[BNX2]: Fix Xen problem.

This fixes the problem of not receiving packets in the Xen bridging
environment.  The Xen script sets the device's MAC address to
FE:FF:FF:FF:FF:FF and puts the device in promiscuous mode.  The
firmware had problem receiving all packets in this configuration.

New firmware and setting the PROM_VLAN bit when in promiscuous mode
will fix this problem.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>