net: cassini: Fix fall-through warnings for Clang

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

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/sun/cassini.c

@@ -1599,6 +1599,7 @@ static inline int cas_mdio_link_not_up(struct cas *cp)
 			cas_phy_write(cp, MII_BMCR, val);
 			break;
 		}
+		break;
 	default:
 		break;
 	}