mt76: set wlan_idx_hi on mt7916

Since mt7916 supports up to 544 wcid entries, wlan_idx_hi needs to be set
in order to support wcid index >= 256

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Bo Jiao <Bo.Jiao@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>