net: ip: packet_socket: fixing raw sckt IPPROTO_RAW usage

net_packet_socket_input() was changed to hardcode the return of
NET_CONTINUE and that caused a segmentation fault/crash in
net_core/process_data(), in cases when pkt was unreferred and
NET_OK was returned from net_conn_input()
This happened with socket combo of: AF_PACKET+SOCK_RAW+IPPROTO_RAW.

Signed-off-by: Jani Hirsimäki <jani.hirsimaki@nordicsemi.no>