ath9k: Handle channel context in get_/set_/reset_tsf

The ath9k TSF handling routines need to be aware of the channel context that
is being modified. With this change the TSF related values that are stored
in each channel context will be correctly tracked and the harware will only
be updated if the modified context is currently the active one.

Without this change the TSF modifications done using these routines would
for example be lost during a hardware reset as done by ath_complete_reset.

Signed-off-by: Benjamin Berg <benjamin.berg@open-mesh.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>