ipheth: remove incorrect devtype to WWAN

The 'wwan' devtype is meant for devices that require preconfiguration
and *every* time setup before the ethernet interface can be used, like
cellular modems which require a series of setup commands on serial ports
or other mechanisms before the ethernet interface will handle packets.

As ipheth only requires one-per-hotplug pairing setup with no
preconfiguration (like APN, phone #, etc) and the network interface is
usable at any time after that initial setup, remove the incorrect
devtype wwan.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>