bfa: Fix for crash when bfa_itnim is NULL

Fix a very corner case when the port gets disconnected and the BFA and
FCS layers clean up references to the IT nexus.  During this window if a
task management command is issued by the SCSI-ML and ends up referencing
a NULL itnim, it could lead to a crash.

Signed-off-by: Sudarsana Kalluru <sudarsana.kalluru@qlogic.com>
Signed-off-by: Anil Gurumurthy <anil.gurumurthy@qlogic.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>