net: sockets: packet: Make it possible to receive packets when unbound

In order to be able to receive packets on unbound packet sockets (which
should collect packet from all interfaces in such case), it's needed to
register receive callback at the socket layer as soon as the socket is
created.

In additional to that, the default binding for packet sockets need to be
revisited. Packet socket should not be bound to the default interface,
as this way the socket would only be receiving packets from that
particular interface. Instead, leave the interface unspecified in such
case.

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