lldp: move lldp_receive_packet() to lldp-internal.c

lldp: move lldp_receive_packet() to lldp-internal.c

In order to implement tests for the LLDP state machine, we need to
mock lldp_network_bind_raw_socket(). Move the other function
lldp_receive_packet() to another file so that we can replace the first
function with a custom one and keep the second one.