ath: lower JP W53 band DFS detection threshold around 30%

Japan's W53 band requires 50% data traffic during its DFS test,
but WLAN baseband used by ath9k and ath10k is not able to achieve
current threshold rate, 50%, under the data traffic rate.
In other words, HW occasionally fails detecting radar pulses,
so that SW cannot get enough radar reports to achieve the rate.

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