Correct rmmod hangs when using HBA disks with write cache enabled.

Do not set controller flag "in_shutdown" during rmmod. SCSI SYNCHRONIZE
CACHE(10) and SCSI SYNCHRONIZE CACHE(16) requests were blocked with
SCSI_MLQUEUE_HOST_BUSY.

Link: https://lore.kernel.org/r/160512627928.2359.10698615071827614781.stgit@brunhildaReviewed-by: Scott Benesh <scott.benesh@microchip.com>
Reviewed-by: Scott Teel <scott.teel@microchip.com>
Signed-off-by: Don Brace <don.brace@microchip.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>