ath9k: Use tsf offset helper in ath9k_hw_reset

These changes make ath9k_hw_reset more consistent with other places that
handle the TSF value by using the same helper routine.

A slight improvement is to not assume that a fixed time of 1.5ms has
passed for the initval writes when compared to the first write attempt.
Instead the TSF value is re-calculated which will yield a higher accuracy
of the restored TSF timer.

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