ath10k: don't reset chip on power_down

Currently hif_power_up performs effectively a
reset and hif_stop resets the chip as well so
there's no point in resetting here.

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