ath9k: Updated Beacon generation to use mac80211-style

This change moves ath9k to use mac80211-generated Beacon frames instead
of trying to allocate a single Beacon frame and then update it. In
addition, the remaining ath_skb_{map,unmap}_single() wrapper calls are
replaced with direct pci_{map,unmap}_single() calls in beacon.c. Power
save buffering for multicast/broadcast frames is not yet converted to
use mac80211-style (frames to be buffered inside mac80211, not in
driver).

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>