ath9k: make beacon timer initialization more reliable

When starting the AP beacon timer, it assumes that the TSF has recently
been cleared. Set the SC_OP_TSF_RESET flag to ensure that this is always
the case.

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