be2net: Add function to issue mailbox cmd on MQ

- Add generic function to issue mailbox cmd on MQ as export function.
- RoCE driver will use this before it setups its own MQ.

Signed-off-by: Parav Pandit <parav.pandit@emulex.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Roland Dreier <roland@purestorage.com>