ath9k: Fix beacon setup

This patch revamps interface addition and deletion and simplifies
slot allocation. There is no need to setup the beacon buffer
in add/remove interface, remove this and use simple APIs for
assigning/deleting slots.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>