wifi: mt76: fix the issue of missing txpwr settings from ch153 to ch177

Because the number of channels to be configured is calculated using the %,
and it results in 0 when there's an exact division, this leads to some
channels not having their tx power configured.

Fixes: 7801da33 ("wifi: mt76: mt7921: enable set txpower for UNII-4")
Signed-off-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>