ath: fix incorrect PPB on JAPAN chirp radar

The number of pulses per burst on Japan chirp radar is
between 1 and 3. The previous value, 20, is representing
number of bursts, but since current DFS detector is using
pulse detection other than bursts, use the pulse number
for correct radar detection.
Also using the highest number helps to avoid false detection.

Signed-off-by: Peter Oh <poh@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>