dissector: arp: Don't convert single byte header fields

There is no need to convert single byte header fields from network to
host order. This even leads to these fields being displayed in a wrong
way.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
dissectors: arp: Display descriptive string for address type fields

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>