bgmac: add ndo_set_rx_mode netdev ops

When changing the device from or to promisc mode this only affects the
device after the device is bought up the next time. For bridging it is
needed to change the device to promisc mode while it is up, which is
possible with this patch.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>