mt76: mt7915: fix tx rate related fields in tx descriptor

Correct the bitfields of tx rateval and bw in tx descriptor.

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