ath10k: implement offset_tsf ieee80211_op

Current set_tsf is implemented in terms of TSF_INCREMENT
only. Instead support new WMI command TSF_DECREMENT and
export these through offset_tsf. Advantage is we get
more accurate TSF adjustments, and don't calculate wrong
offset in case absolute TSF was calculated from rx_mactime
(actual TSF).

The new WMI command is available in firmware
10.4-3.2.1-00033 for QCA4019 chips. Old drivers on new
firmware or vice versa shouldn't  be a problem since
get/set tsf logic was already broken.

Signed-off-by: Thomas Pedersen <twp@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>