interfaces: only register protocol handler for LLDP when only LLDP enabled

interfaces: only register protocol handler for LLDP when only LLDP enabled

On Linux, the drop counter is increased on unhandled packets. We are
using a raw socket with ETH_P_ALL, so we get a copy of the packet. The
original packet is ultimately dropped later and this increases the
drop counter associated to the interface on Linux.

When listening only to LLDP, use ETH_P_LLDP instead of ETH_P_ALL to
avoid this.

Fix #296.