scsi: qla2xxx: Prevent multiple ADISC commands per session

Add check to allow 1 discovery command per session to be sent.

Signed-off-by: Quinn Tran <qtran@marvell.com>
Signed-off-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>