scsi: ufshcd: Flush exception event before suspend

Exception event can be raised by the device when system
suspend is in progress. This will result in unclocked
register access in exception event handler as clocks will
be turned off during suspend. This change makes sure to flush
exception event handler work in suspend before disabling
clocks to avoid unclocked register access issue.

Change-Id: I5a089a0387790a12f4cc3213c75e9677467a2a07
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>