If CONFIG_PREEMPT=y:

    Unable to handle kernel NULL pointer dereference at virtual address 00000014
    pgd = c0003000
    [00000014] *pgd=80000040004003, *pmd=00000000
    Internal error: Oops: 206 [#1] PREEMPT SMP ARM
    Modules linked in:
    CPU: 0 PID: 17 Comm: kworker/0:1 Tainted: G        W       4.3.0-rc3-koelsch-022
    71-g705498fc5e6a5da8-dirty #1789
    Hardware name: Generic R8A7791 (Flattened Device Tree)
    Workqueue: pm pm_runtime_work
    task: ef578e40 ti: ef57a000 task.ti: ef57a000
    PC is at usb_dmac_chan_halt+0xc/0xc0
    LR is at usb_dmac_runtime_suspend+0x28/0x38
    pc : [<c023c880>]    lr : [<c023c95c>]    psr: 80000113
    sp : ef57bdf8  ip : 00000008  fp : 00000003
    r10: 00000008  r9 : c06ab928  r8 : ef49e810
    r7 : 00000000  r6 : 000000ac  r5 : ef770010  r4 : 00000000
    r3 : 00000000  r2 : 8ffc2b84  r1 : 00000000  r0 : ef770010
    Flags: Nzcv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
    Control: 30c5307d  Table: 40003000  DAC: fffffffd
    Process kworker/0:1 (pid: 17, stack limit = 0xef57a210)
    Stack: (0xef57bdf8 to 0xef57c000)

    [...

    [<c023c880>] (usb_dmac_chan_halt) from [<c023c95c>] (usb_dmac_runtime_suspend+0x28/0x38)
    [<c023c95c>] (usb_dmac_runtime_suspend) from [<c027b25c>] (pm_genpd_runtime_suspend+0x74/0x23c)

This happens because usb_dmac_probe() calls pm_runtime_put() before
usb_dmac_chan_probe(), leading to the device being suspended before the
DMA channels are initialized, causing a NULL pointer dereference.

Move the call to pm_runtime_put() to the end of usb_dmac_probe() to fix
this.

Add a check to usb_dmac_runtime_suspend() to prevent the crash from
happening in the error path.
Reported-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>