net: pkt: Add ll_proto_type field

The L2 protocol type information is not carried to the upper layers.
This is problematic for packet sockets, as the address structure in
recvfrom() is supposed to provide this information.

Fix this by adding ll_proto_type field in the net_pkt structure.
Set the protocol type in the Ethernet L2 when packet is processed.

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