Phonet: return an error when packet TX fails

Phonet assumes that packets are never dropped. We try our best to
avoid this situation. But lets return ENOBUFS if queueing to the
network device fails so that the caller knows things went wrong.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>