[SCSI] qla4xxx: Correct the validation to check in get_sys_info mailbox

Check mbox_sts[3] instead of mbox_sts[4] for ISP83xx to validate size
of data returned

Signed-off-by: Nilesh Javali <nilesh.javali@qlogic.com>
Signed-off-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>