net: macb: 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>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/macb_ptp.c

@@ -466,6 +466,7 @@ int gem_set_hwtst(struct net_device *dev, struct ifreq *ifr, int cmd)
 	case HWTSTAMP_TX_ONESTEP_SYNC:
 		if (gem_ptp_set_one_step_sync(bp, 1) != 0)
 			return -ERANGE;
+		/* fall through */
 	case HWTSTAMP_TX_ON:
 		tx_bd_control = TSTAMP_ALL_FRAMES;
 		break;