wwan_hwsim: support network interface creation

Add support for networking interface creation via the WWAN core by
registering the WWAN netdev creation ops for each simulated WWAN device.
Implemented minimalistic netdev support where the xmit callback just
consumes all egress skbs.

This should help with WWAN network interfaces creation testing.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>