net: l2: ppp: Make PPP L2 work with packet sockets

Currently, the L2 PPP won't work with AF_PACKET socket family as it only
supports packets from AF_INET/AF_INET6 families. Because of this, it's
not possible to use AF_PACKET RAW or DGRAm sockets with PPP interfaces,
as the packets they generate have family field set to AF_PACKET.

Fix this, by verifying the LL protocol field in the PPP L2 before
passing the packet the respective PPP driver. If the AF_PACKET packet is
received, and the protocol field is set to IP/IPv6, update the packet
family to AF_INET/AF_INET6 accordingly.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>