wifi: mt76: add tx_nss histogram to ethtool stats

mt76_connac2_mac_fill_txs() counts tx_nss but ethtool doesn't show
stats. Add missing histogram accordingly.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>