User can trigger disabling of gadget at run time while the
transfers are going on.
Eg: 1: rmmod of musb driver while transfers are going on

Eg: 2: On android doing:
 echo 0       > /sys/class/android_usb/android0/enable
While a big file transfer is going on via PTP/MTP.

In such a case, musb_gadget_disable() calls nuke()
but the dma interrupt may still happen for an endpoint since hw
would raise the interrupt in anycase.

This can result in a NULL pointer access crash:

[  314.030426] PC is at txstate+0x74/0x20c
[  314.034759] LR is at musb_g_tx+0x140/0x204
[  314.039489] pc : [<c03506f4>]    lr : [<c0350bcc>]    psr: 20000193
[  314.039520] sp : c783bc68  ip : 00000002  fp : c783bc9c
[  314.052429] r10: 00000018  r9 : 00000000  r8 : 00000200
[  314.058258] r7 : 00000000  r6 : fc0ab130  r5 : c781a410  r4 : c6caf640
[  314.065643] r3 : 00000000  r2 : 00000000  r1 : 00000000  r0 : c781a000
[  315.083251] Backtrace:
[  315.086242] [<c0350680>] (txstate+0x0/0x20c) from [<c0350bcc>] (musb_g_tx+0x140/0x204)
[  315.095123] [<c0350a8c>] (musb_g_tx+0x0/0x204) from [<c034eb00>] (musb_dma_completion+0x40/0x54)
[  315.104980] [<c034eac0>] (musb_dma_completion+0x0/0x54) from [<c0351e6c>] (dma_controller_irq+0x118/0x184)
[  315.115661] [<c0351d54>] (dma_controller_irq+0x0/0x184) from [<c00d86b8>] (handle_irq_event_percpu+0x54/0x188)

So put protection in code to handle possiblity of getting an interrupt for an
endpoint that might have been already nuked.
Reported-by: NTodd Poynor <toddpoynor@google.com>
Signed-off-by: NVikram Pandita <vikram.pandita@ti.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>