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>