ath9k: fix ad-hoc nexttbtt calculation

rounding up the delta between last-beacon-tsf and tsf to intval is wrong
and can lead to misconfigured timers which breaks beacon transmission.
Fix this by adding intval and subtracting the offset of the tsf within the
current slot.

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