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.

---
Changes in RESEND:
- None. Resending now that net-next is open.

 drivers/net/ethernet/sun/cassini.c | 1 +
 1 file changed, 1 insertion(+)

index 9ff894ba8d3e..54f45d8c79a7 100644
@@ -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;
 	}

2.27.0