NEC plugin: excludes router interfaces from apply_ports

PFC v51 packet filter does not support a filter for vRouter interface.
This patch checks ofc_port_id in _extract_ofc_port_id in more strict
way and exclude non-normal OFC ports (i.e., router interfaces at now)
from apply_ports of create_ofc_filter.

The same change also applies to other _extract_ofc_*_id methods in
PFC drvier.