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:
   ^~~~~~~

Signed-off-by: Gustavo A. R. Silva <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

drivers/net/ethernet/smsc/smc911x.c

@@ -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);