net: sockets: packet: Allow proto 0 for RAW sockets

According to AF_PACKET man pages protocol number 0 is allowed, however
in such case the socket is only capable of transmitting packets then:

  "If protocol is set to zero, no packets are received."

Therefore, allow to create sockets with such protocol, and at the
connection.c level filter out such sockets from data reception.

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