ath10k: add debugfs entry to configure quiet period

Add support to configure quiet period (in milliseconds) via debugfs.
This is useful to experiment different quiet period values along with
different throttle ratio.

echo 100 > /sys/kernel/debug/ieee80211/phyX/ath10k/quiet_period

Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>