ath5k: remove unused led_off parameter

ath5k_softc->led_off hasn't been used since commit
3a078876, "convert LED code to use
mac80211 triggers."

Changes-licensed-under: 3-Clause-BSD

Reported-by: Tobias Doerffel <tobias.doerffel@gmail.com>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>