vxlan: implement get_link_ksettings ethtool method

Similar to VLAN and similar drivers, we can forward get_link_ksettings to
the lower dev if we have one to get meaningful speed/duplex data.

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