don't hide the mtu on "bridge" interfaces.

don't hide the mtu on "bridge" interfaces.

interfaces like vxlan and nvgre have bridges inside them and respond
to bridge ioctls, but they are still interfaces that handle l3
traffic so the mtu means something on them. if we don't want bridge
to show an mtu, that can be done by bridge(4) instead of having
ifconfig make assumptions like this.

noticed by jason tubnor
ok deraadt@ claudio@