scsi: qla2xxx: Update rscn_rcvd field to more meaningful scan_needed

Rename rscn_rcvd field to scan_needed to be more meaningful.

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>