scsi: ufs: avoid exception event handler racing with PM callbacks

If device raises the exception event in the response to the commands
sent during the runtime/system PM callbacks, exception event handler
might run in parallel with PM callbacks and may see unclocked register
accesses. This change fixes this issue by not scheduling the exception
event handler while PM callbacks are running.

Change-Id: I8644f50366d5c87ba6a5048e48703b3723ab241b
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>