NEC plugin: Allow users to clear packet filter fields

Change-Id: Ia03b510b9b473b98b6eca85578cd80ec729ef001
NEC plugin: Fix comparison operator in trema filter driver

Change-Id: I846a096c4a66680a7f35ed934775c620ce27b71a
NEC plugin: trema driver should not have update_filter method

packet_fitler mixin determines if a backend supports update_filter
by checking if a driver has update_filter method.
Packet filter implementation of Trema Sliceable Switch does not
support update_filter, so the driver should define it.

Change-Id: I2f8f65c324b8f623b26aabf0385818565f3e1b8d
NEC plugin (packet filter): Update eth_type based on protocol

Remove the corresponding logic from drivers

Change-Id: I0e7db085d8183c08d923e20ba0b2c8b6c6db58e7