ath9k_hw: disable hardware ad-hoc flag on ar934x rev 3

On AR934x rev 3, settin the ad-hoc flag completely messes up hardware
state - beacons get stuck, almost no packets make it out, hardware is
constantly reset.

When leaving out that flag and setting up the hw like in AP mode, TSF
timers won't be automatically synced, but at least the rest works.

AR934x rev 2 and older are not affected by this bug

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>