Phonet: correct pipe backlog callback return values

In some cases, the Phonet pipe backlog callbacks returned negative
errno instead of NET_RX_* values.

In other cases, NET_RX_DROP was returned for invalid packets, even
though it seems only intended for buffering problems (not for
deliberately discarded packets).

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