ath9k_hw: fix TSF save/restore around chip reset

A cold reset can be triggered because of DMA stop issues, and this leads
to TSF being cleared on all chipsets. To properly deal with this, always
save the TSF.
Additionally, account for the time it takes to do the actual chip reset,
which can be quite significant. On AR9344 it takes around 4.5 ms.

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