bridge: implement get_link_ksettings ethtool method

We return the maximum speed of all active ports. This matches how the link
speed would give an upper limit for traffic to/from any single peer if the
bridge were replaced with a hardware switch.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: David S. Miller <davem@davemloft.net>