Fixes issues with scanning and low power output at some rates.

Fixes: f75e4779 ("wifi: mt76: mt7996: add txpower setting support")
Signed-off-by: Chad Monroe <chad@monroe.io>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>