ath9k: fix beacon timer restart after a card reset

In AP mode, ath_beacon_config_ap only restarts the timer if a TSF
restart is requested. Apparently this was added, because this function
unconditionally sets the flag for TSF reset.

The problem with this is, that ath9k_hw_reset() clobbers the timer
registers (specified in the initvals), thus effectively disabling the
SWBA interrupt whenever a card reset without TSF reset is issued
(happens in a few places in the code).

This patch fixes ath_beacon_config_ap to only issue the TSF reset flag
when necessary, but reinitialize the timer unconditionally. Tests show,
that this is enough to keep the SWBA interrupt going after a call to
ath_reset()

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