net: smc911x: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: arm):

drivers/net/ethernet/smsc/smc911x.c: In function ‘smc911x_phy_detect’:
drivers/net/ethernet/smsc/smc911x.c:677:7: warning: this statement may fall through [-Wimplicit-fallthrough=]

    if (cfg & HW_CFG_EXT_PHY_DET_) {
       ^

drivers/net/ethernet/smsc/smc911x.c:715:3: note: here
default:
^~~~~~~

---
 drivers/net/ethernet/smsc/smc911x.c | 1 +
 1 file changed, 1 insertion(+)

index bd14803545de..8d88e4083456 100644
@@ -712,6 +712,7 @@ static void smc911x_phy_detect(struct net_device *dev)
 					/* Found an external PHY */
 					break;
 			}
+			/* Else, fall through */
 		default:
 			/* Internal media only */
 			SMC_GET_PHY_ID1(lp, 1, id1);

2.22.0

From: "Gustavo A. R. Silva" <[email protected]>
Date: Mon, 29 Jul 2019 17:10:16 -0500

Applied.