ath9k: Reset SC_OP_TSF_RESET flag after stuck beacon

I have a TrendNet 652-BRP running OpenWRT + ath9k very well.  The only
problem is that the beacon gets stuck maybe once a day.  After
Vasanthakumar Thiagarajan's "ath9k: cleanup beacon parameters
configuration" patch, ath9k would nearly re-configure the beacons after it
detected the stuck beacon, and did a reset.  But it would fail the
SC_OP_TSF_RESET check in ath_beacon_config_ap.  This patch gets the beacon
fully reconfigured after the reset.

Signed-off-by: Jeff Hansen <x@jeffhansen.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>