l2tp: remove useless device duplication test in l2tp_eth_create()

There's no need to verify that cfg->ifname is unique at this point.
register_netdev() will return -EEXIST if asked to create a device with
a name that's alrealy in use.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>