mt7601u: phy: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/mediatek/mt7601u/phy.c

@@ -795,6 +795,7 @@ mt7601u_phy_rf_pa_mode_val(struct mt7601u_dev *dev, int phy_mode, int tx_rate)
 	switch (phy_mode) {
 	case MT_PHY_TYPE_OFDM:
 		tx_rate += 4;
+		/* fall through */
 	case MT_PHY_TYPE_CCK:
 		reg = dev->rf_pa_mode[0];
 		break;