scsi: megaraid_sas: unload flag should be set after scsi_remove_host is called

Issue - Driver returns DID_NO_CONNECT when unload is in progress,
indicated using instance->unload flag. In case of dynamic unload of
driver, this flag is set before calling scsi_remove_host(). While doing
manual driver unload, user will see lots of prints for Sync Cache
command with DID_NO_CONNECT status.

Fix - Set the instance->unload flag after scsi_remove_host(). Allow
device removal process to be completed and do not block any command
before that.  SCSI commands (like SYNC_CACHE) are received (as part of
scsi_remove_host) by driver during unload will be submitted further down
to the drives.

Signed-off-by: Sumit Saxena <sumit.saxena@broadcom.com>
Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>