[SCSI] bfa: Fix to process mbox interrupts only if interrupts are enabled in INTx mode.

- Made changes to process mbox and error interrupts only if interrupts are
  enabled, when running in INTx mode.

Signed-off-by: Krishna Gudipati <kgudipat@brocade.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>