usb: gadget: udc: Flush pending work also in error path

When binding an UDC driver to the pending gadget fails in
check_pending_gadget_drivers(), the usb_add_gadget_udc_release() function
ends without waiting for the usb_gadget_state_work to finish, what in
turn might cause the whole struct usb_gadget being freed by the caller
before the usb_gadget_state_work being executed.

This can be observed on some boards with USB Mass Storage gadget
compiled-in and kernel booted without the needed module parameters:

dwc2 12480000.hsotg: dwc2_check_params: Invalid parameter besl=1
dwc2 12480000.hsotg: dwc2_check_params: Invalid parameter g_np_tx_fifo_size=1024
dwc2 12480000.hsotg: EPs: 16, dedicated fifos, 7808 entries in SPRAM
Mass Storage Function, version: 2009/09/11
LUN: removable file: (no medium)
no file given for LUN0
g_mass_storage 12480000.hsotg: failed to start g_mass_storage: -22
dwc2: probe of 12480000.hsotg failed with error -22
8<--- cut here ---
Unable to handle kernel NULL pointer dereference at virtual address 00000004
pgd = (ptrval)
[00000004] *pgd=00000000
Internal error: Oops: 5 [#1] PREEMPT SMP ARM
Modules linked in:
CPU: 1 PID: 88 Comm: kworker/1:2 Not tainted 5.8.0-rc5-next-20200715-00062-gc5bb489ae825-dirty #8792
Hardware name: Samsung Exynos (Flattened Device Tree)
Workqueue:  0x0 (rcu_gp)
PC is at process_one_work+0x44/0x7dc
...
Process kworker/1:2 (pid: 88, stack limit = 0x(ptrval))
Stack: (0xed9f1f00 to 0xed9f2000)
...
[<c0148590>] (process_one_work) from [<c0148d6c>] (worker_thread+0x44/0x51c)
[<c0148d6c>] (worker_thread) from [<c01500c0>] (kthread+0x158/0x1a0)
[<c01500c0>] (kthread) from [<c0100114>] (ret_from_fork+0x14/0x20)
Exception stack(0xed9f1fb0 to 0xed9f1ff8)
...
---[ end trace 5033c1326a62e5f3 ]---
note: kworker/1:2[88] exited with preempt_count 1

Fix this by flushing pending work in error path.
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/gadget/udc/core.c

@@ -1230,6 +1230,7 @@ int usb_add_gadget_udc_release(struct device *parent, struct usb_gadget *gadget,
 	return 0;
 
  err_del_udc:
+	flush_work(&gadget->work);
 	device_del(&udc->dev);
 
  err_unlist_udc: