mt76: mt7915: fix IEEE80211_HE_PHY_CAP7_MAX_NC for station mode

The value of station mode is always 0.

Fixed: 00b2e16e ("mt76: mt7915: add TxBF capabilities")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>