ath9k: Disable staggered Beacon frame scheduling

It looks like there are some issues in the current ath9k version as
far as staggered Beacon frame scheduling is concerned. This results in
Beacon frame timestamp being off by 25 milliseconds or so which can
cause issues with multicast power save buffering in AP mode. Some
client stations fail to receive multicast frames when there is this
large an offset between TBTT and actual Beacon transmission time.

Since ath9k does not yet support multiple BSSes anyway, the staggered
Beacon frame scheduling is just increasing the number of interrupts at
this point. The easiest fix for the timestamp offset is to disable
staggered Beacon frames for now. We need to redesign beacon.c anyway
when adding multi-BSS support with mac80211 since the existing code
depends on different mechanism for Beacon frame transmission.

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