scsi: qla2xxx: Serialize GPNID for multiple RSCN

GPNID is triggered by RSCN. For multiple RSCNs of the same affected
NPORT ID, serialize the GPNID to prevent confusion.

Fixes: 726b8548 ("qla2xxx: Add framework for async fabric discovery")
Cc: <stable@vger.kernel.org> # 4.10+
Signed-off-by: Quinn Tran <quinn.tran@cavium.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>