scsi: qla2xxx: Avoid that Coverity complains about dereferencing a NULL rport pointer

Since Coverity cannot know that rport != NULL in qla2xxx_queuecommand() and
since there is code in that function that dereferences the rport pointer,
modify qla2xxx_queuecommand() such that it fails SCSI commands if rport ==
NULL.

Cc: Himanshu Madhani <hmadhani@marvell.com>
Cc: Giridhar Malavali <gmalavali@marvell.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>