ath9k: fix beacon timer handling issues

AP mode beacon timers in ath9k are configured in milliseconds, which breaks
when increasing ATH_BCBUF to 8 instead of 4 (due to rounding errors).
Since the hardware timers are actually configured in microseconds, it's
better to let the driver use that unit directly.

To be able to do that, the beacon interval parameter abuse for passing
certain flags needs to be removed. This is easy to do, because those flags
are completely unnecessary anyway. ATH9K_BEACON_ENA is ignored,
ATH9K_BEACON_RESET_TSF can be replaced with calling ath9k_hw_reset_tsf
from the driver directly.

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