netxen: fix promisc mode, mtu setting

For NX3031, multicast filtering, promisc mode, and max frame size
setting is handled by firmware, driver needs to send request to
enable/disable it.

For old chip revisions / firmware, driver still sets it directly.

Added function pointer to set mtu according to chip revision.

Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>