openvswitch: Warn over-mtu packets only if iface is UP.

It is not unusual to have the bridge port down. Sometimes
it has the old MTU, which is fine since it's not being used.

However, the kernel spams the log with a warning message
when a packet is going to be sent over such port. Fix that
by warning only if the interface is UP.

Signed-off-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: David S. Miller <davem@davemloft.net>