ath9k: use the hw opmode to select the beacon timer mode

Since the beacon timers are global, the individual vif type should not
be used to determine the beacon timer configuration mode, use the
global opmode instead.

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