vxlan: move Ethernet initialization to a separate function

This will allow to initialize vxlan in ARPHRD_NONE mode based on the passed
rtnl attributes.

v2: renamed "l2mode" to "ether".

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>