ath10k: fix STA u-APSD

To comply with WMM-PS the device shouldn't wake up
with a NullFunc frame pair when tx-ing. Instead PM
bit on each tx frame should be used.

To make this work correctly firmware needs to be
told to use a different STA PS wake threshold when
u-APSD is enabled.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>