mt76: mt7921: fixup rx bitrate statistics

Since the related rx bitrate fields have been moved to group3 in Rxv,
fix rx bitrate statistics in mt7921_mac_fill_rx routine.

Fixes: 163f4d22 ("mt76: mt7921: add MAC support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>