drivers/usb: Delete XHCI command timer if necessary

When xhci_mem_cleanup() is called, it's possible that the command
timer isn't initialized and scheduled. For those cases, to delete
the command timer causes soft-lockup as below stack dump shows.

The patch avoids deleting the command timer if it's not scheduled
with the help of timer_pending().

NMI watchdog: BUG: soft lockup - CPU#40 stuck for 23s! [kworker/40:1:8140]
      :
NIP [c000000000150b30] lock_timer_base.isra.34+0x90/0xa0
LR [c000000000150c24] try_to_del_timer_sync+0x34/0xa0
Call Trace:
[c000000f67c975e0] [c0000000015b84f8] mon_ops+0x0/0x8 (unreliable)
[c000000f67c97620] [c000000000150c24] try_to_del_timer_sync+0x34/0xa0
[c000000f67c97660] [c000000000150cf0] del_timer_sync+0x60/0x80
[c000000f67c97690] [c00000000070ac0c] xhci_mem_cleanup+0x5c/0x5e0
[c000000f67c97740] [c00000000070c2e8] xhci_mem_init+0x1158/0x13b0
[c000000f67c97860] [c000000000700978] xhci_init+0x88/0x110
[c000000f67c978e0] [c000000000701644] xhci_gen_setup+0x2b4/0x590
[c000000...