[SCSI] qla2xxx: Clean up qla84xx_mgmt_cmd()

Remove dead code, simplify a pointer computation and move the
ql84_mgmt assignment to just before its first use.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Saurav Kashyap <saurav.kashyap@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>