usb: musb: Fix crash when dma is nuked while irq handler is pending

It appears possible for the dma to get nuked while the irq handler
is pending/about to run.  The dma is cancelled by nuke() but the
irq handler will still run and wind up dereferencing NULL, so add
a check.

Crash log:

[ 7629.098815] Unable to handle kernel NULL pointer dereference at virtual address 00000004
[ 7629.099273] pgd = c0004000
[ 7629.099456] [00000004] *pgd=00000000
[ 7629.099761] Internal error: Oops: 17 [#1] PREEMPT SMP
[ 7629.100067] Modules linked in:
[ 7629.100402] CPU: 0    Not tainted  (3.0.8-gcb25bf2 #1)
[ 7629.100585] PC is at txstate+0x74/0x1ec
[ 7629.100891] LR is at musb_g_tx+0x138/0x1d8
[ 7629.101074] pc : [<c033f1cc>]    lr : [<c033fc9c>]    psr: 20000193
[ 7629.101074] sp : c06efd60  ip : c06efd98  fp : c06efd94
[ 7629.101531] r10: 00000018  r9 : 00000000  r8 : 00000200
[ 7629.101684] r7 : 00000000  r6 : fc0ab110  r5 : c7852448  r4 : c7125c00
[ 7629.101867] r3 : 00000000  r2 : 00000000  r1 : 00000002  r0 : c78520e4
...
[ 7629.228668] [<c033f158>] (txstate+0x0/0x1ec) from [<c033fc9c>] (musb_g_tx+0x138/0x1d8)
[ 7629.229003] [<c033fb64>] (musb_g_tx+0x0/0x1d8) from [<c033d35c>] (musb_interrupt+0x104/0x8cc)
[ 7629.229187] [<c033d258>] (musb_interrupt+0x0/0x8cc) from [<c033db7c>] (generic_interrupt+0x58/0x8c)
[ 7629.229522] [<c033db24>] (generic_interrupt+0x0/0x8c) from [<c00da008>] (handle_irq_event_percpu+0x54/0x188)
[ 7629.229827]  r6:00000000 r5:c06f3f4c r4:c78f49c0 r3:c033db24

Change-Id: I305c80a246f5161f2d3d20839c5603376d5b746c
Signed-off-by: Mike J. Chen <mjchen@google.com>
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@ti.com>