net: axienet: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly adding a fallthrough statement instead of just
letting the code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210528195831.GA39131@embeddedor
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/xilinx/xilinx_axienet_main.c

@@ -1543,6 +1543,7 @@ static void axienet_validate(struct phylink_config *config,
 	case PHY_INTERFACE_MODE_MII:
 		phylink_set(mask, 100baseT_Full);
 		phylink_set(mask, 10baseT_Full);
+		fallthrough;
 	default:
 		break;
 	}