bnx2x: Ack unknown VF messages

A PF should ack the firmware when receiving unknown messages through the VF's
mailbox. This prevents the VF mailbox from being stuck in case of a VF sending
a message unknown to the PF (e.g. VF with more advanced version).

Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>