ath5k: fix recursive locking in ath5k_beacon_update

ath5k_beacon_update takes sc->lock upon entry.  However, it is only
called from within ath5k_config_interface, which already holds the lock.
Remove the unnecessary locking from ath5k_beacon_update.

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