mt76: mt7915: set peer Tx fixed rate through debugfs

Driver can manually set fixed rate for each peer through debugfs.

May use .set_bitrate_mask callback and iterate stations under the
current vif to achieve the aim once it supports HE rate.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>