scsi: qla2xxx: Set the qpair in SRB to NULL when SRB is released

This patch sets QPair pointer to NULL to prevent abort command racing ahead
of normal command completion handling during scsi_done call.

Signed-off-by: Giridhar Malavali <gmalavali@marvell.com>
Signed-off-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>