ath9k: properly preserve TSF across reset

The beacon code previously reset TSF on every configuration call, as
some of the code was not prepared to properly calculate nexttbtt based
on current TSF.

This patch adds a common function for calculating nexttbtt and moves the
TSF reset to driver start.

This should improve AP mode compatibility with various stations that
expect the TSF to not randomly jump due to hardware resets.

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