Add wait for session deletion to finish before freeing an NPIV scsi host.

Fixes: 726b8548 ("qla2xxx: Add framework for async fabric discovery")
Cc: <stable@vger.kernel.org>
Signed-off-by: NQuinn Tran <quinn.tran@cavium.com>
Signed-off-by: NHimanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>