ath5k: Correct usage of AR5K_CFG_ADHOC

This corrects usage of AR5K_CFG_ADHOC introduced in
"ath5k: Update PCU code". Also,
the name of the indicator is changed to AR5K_CFG_IBSS to more
accurately reflect its function. This change restores
beaconing in AP and mesh modes.

Signed-off-by: Steve Brown <sbrown@cortland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>