mt76: mt7915: fix mcs_map in mt7915_mcu_set_sta_he_mcs()

Should use peer's bandwidth instead of chandef->width to
get correct mcs_map.

Fixes: 76be6c07 ("mt76: mt7915: add .set_bitrate_mask() callback")
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>