qlcnic: Fix mailbox processing during diagnostic test

o Do not enable mailbox polling in case of legacy interrupt.
  Process mailbox AEN/response from the interrupt.

Signed-off-by: Manish Chopra <manish.chopra@qlogic.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>