Currently, there's no guarantee that udc->driver
will be valid when using soft_connect sysfs
interface. In fact, we can very easily trigger
a NULL pointer dereference by trying to disconnect
when a gadget driver isn't loaded.

Fix this bug:

~# echo disconnect > soft_connect
[   33.685743] Unable to handle kernel NULL pointer dereference at virtual address 00000014
[   33.694221] pgd = ed0cc000
[   33.697174] [00000014] *pgd=ae351831, *pte=00000000, *ppte=00000000
[   33.703766] Internal error: Oops: 17 [#1] SMP ARM
[   33.708697] Modules linked in: xhci_plat_hcd xhci_hcd snd_soc_davinci_mcasp snd_soc_tlv320aic3x snd_soc_edma snd_soc_omap snd_soc_evm snd_soc_core dwc3 snd_compress snd_pcm_dmaengine snd_pcm snd_timer snd lis3lv02d_i2c matrix_keypad lis3lv02d dwc3_omap input_polldev soundcore
[   33.734372] CPU: 0 PID: 1457 Comm: bash Not tainted 3.17.0-09740-ga93416e-dirty #345
[   33.742457] task: ee71ce00 ti: ee68a000 task.ti: ee68a000
[   33.748116] PC is at usb_udc_softconn_store+0xa4/0xec
[   33.753416] LR is at mark_held_locks+0x78/0x90
[   33.758057] pc : [<c04df128>]    lr : [<c00896a4>]    psr: 20000013
[   33.758057] sp : ee68bec8  ip : c0c00008  fp : ee68bee4
[   33.770050] r10: ee6b394c  r9 : ee68bf80  r8 : ee6062c0
[   33.775508] r7 : 00000000  r6 : ee6062c0  r5 : 0000000b  r4 : ee739408
[   33.782346] r3 : 00000000  r2 : 00000000  r1 : ee71d390  r0 : ee664170
[   33.789168] Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
[   33.796636] Control: 10c5387d  Table: ad0cc059  DAC: 00000015
[   33.802638] Process bash (pid: 1457, stack limit = 0xee68a248)
[   33.808740] Stack: (0xee68bec8 to 0xee68c000)
[   33.813299] bec0:                   0000000b c0411284 ee6062c0 00000000 ee68bef4 ee68bee8
[   33.821862] bee0: c04112ac c04df090 ee68bf14 ee68bef8 c01c2868 c0411290 0000000b ee6b3940
[   33.830419] bf00: 00000000 00000000 ee68bf4c ee68bf18 c01c1a24 c01c2818 00000000 00000000
[   33.838990] bf20: ee61b940 ee2f47c0 0000000b 000ce408 ee68bf80 c000f304 ee68a000 00000000
[   33.847544] bf40: ee68bf7c ee68bf50 c0152dd8 c01c1960 ee68bf7c c0170af8 ee68bf7c ee2f47c0
[   33.856099] bf60: ee2f47c0 000ce408 0000000b c000f304 ee68bfa4 ee68bf80 c0153330 c0152d34
[   33.864653] bf80: 00000000 00000000 0000000b 000ce408 b6e7fb50 00000004 00000000 ee68bfa8
[   33.873204] bfa0: c000f080 c01532e8 0000000b 000ce408 00000001 000ce408 0000000b 00000000
[   33.881763] bfc0: 0000000b 000ce408 b6e7fb50 00000004 0000000b 00000000 000c5758 00000000
[   33.890319] bfe0: 00000000 bec2c924 b6de422d b6e1d226 40000030 00000001 75716d2f 00657565
[   33.898890] [<c04df128>] (usb_udc_softconn_store) from [<c04112ac>] (dev_attr_store+0x28/0x34)
[   33.907920] [<c04112ac>] (dev_attr_store) from [<c01c2868>] (sysfs_kf_write+0x5c/0x60)
[   33.916200] [<c01c2868>] (sysfs_kf_write) from [<c01c1a24>] (kernfs_fop_write+0xd0/0x194)
[   33.924773] [<c01c1a24>] (kernfs_fop_write) from [<c0152dd8>] (vfs_write+0xb0/0x1bc)
[   33.932874] [<c0152dd8>] (vfs_write) from [<c0153330>] (SyS_write+0x54/0xb0)
[   33.940247] [<c0153330>] (SyS_write) from [<c000f080>] (ret_fast_syscall+0x0/0x48)
[   33.948160] Code: e1a01007 e12fff33 e5140004 e5143008 (e5933014)
[   33.954625] ---[ end trace f849bead94eab7ea ]---

Fixes: 2ccea03a (usb: gadget: introduce UDC Class)
Cc: <stable@vger.kernel.org> # v3.1+
Signed-off-by: Felipe Balbi <balbi@ti.com>