ath10k: fix duration calculation for quiet param

The duty cycle (% of quiet duration) is used to put the device
in quiet mode for the given period. Currently the quiet duration
is wrongly calculated which results in not enabling quiet mode.
Fix the calculation as below

     duration = (period * duty cycle) / 100

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