net: 3c509: mark expected switch fall-throughs

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

This patch fixes the following warnings:

drivers/net/ethernet/3com/3c509.c:1265:8: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/net/ethernet/3com/3c509.c:1271:8: warning: this statement may fall through [-Wimplicit-fallthrough=]

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/3com/3c509.c

@@ -1266,12 +1266,14 @@ el3_up(struct net_device *dev)
 					pr_cont("Forcing 3c5x9b full-duplex mode");
 					break;
 				}
+				/* fall through */
 			case 8:
 				/* set full-duplex mode based on eeprom config setting */
 				if ((sw_info & 0x000f) && (sw_info & 0x8000)) {
 					pr_cont("Setting 3c5x9b full-duplex mode (from EEPROM configuration bit)");
 					break;
 				}
+				/* fall through */
 			default:
 				/* xcvr=(0 || 4) OR user has an old 3c5x9 non "B" model */
 				pr_cont("Setting 3c5x9/3c5x9B half-duplex mode");