mt76: mt7921: fix firmware usage of RA info using legacy rates

According to the firmware usage, OFDM rates should fill out bit 6 - 13
while CCK rates should fill out bit 0 - 3 in legacy field of RA info to
make the rate adaption runs propertly. Otherwise, a unicast frame might be
picking up the unsupported rate to send out.

Fixes: 1c099ab4 ("mt76: mt7921: add MCU support")
Reported-by: Joshua Emele <jemele@chromium.org>
Co-developed-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>