l2tp: define "l2tpeth" device type

Export type of l2tpeth interfaces to userspace
(/sys/class/net/<iface>/uevent).

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Acked-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>