ath5k: convert LED code to use mac80211 triggers

This change cleans up the ath5k LED code and converts it to use
the standard LED device class along with the rx/tx LED triggers
provided by mac80211.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>