wifi: mt76: mt7915: also MT7981 is 3T3R but nss2 on 5 GHz band

Just like MT7916 also MT7981 can handle 3T3R DBDC frontend and should
hence be included in the corresponding conditional expression in the
driver. Add it.

Fixes: 6bad146d ("wifi: mt76: mt7915: add support for MT7981")
Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>