[SCSI] qla4xxx: Don't process mbx interrupt unconditionally

Do not process interrupt unconditionally during mailbox processing  which can
lead to spurious interrupt. Mailbox completion are now polled if interrupt are
disabled or wait for interrupt to come in if its enabled

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>