net: ksz884x: Use fallthrough pseudo-keyword

Replace /* Fallthrough... */ comment with the new pseudo-keyword macro
fallthrough[1].

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/micrel/ksz884x.c

@@ -5833,8 +5833,7 @@ static int netdev_ioctl(struct net_device *dev, struct ifreq *ifr, int cmd)
 	/* Get address of MII PHY in use. */
 	case SIOCGMIIPHY:
 		data->phy_id = priv->id;
-
-		/* Fallthrough... */
+		fallthrough;
 
 	/* Read MII PHY register. */
 	case SIOCGMIIREG: