mdio: mux: fix an incorrect less than zero error check using a u32

The u32 variable v is being checked to see if an error return is
less than zero and this check has no effect because it is unsigned.
Fix this by making v and int (this also matches the type of
cb->bus_number which is assigned to the value in v).

Detected by CoverityScan, CID#1440454 ("Unsigned compared against zero")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/mdio-mux.c

@@ -133,7 +133,7 @@ int mdio_mux_init(struct device *dev,
 
 	ret_val = -ENODEV;
 	for_each_available_child_of_node(dev->of_node, child_bus_node) {
-		u32 v;
+		int v;
 
 		v = of_mdio_parse_addr(dev, child_bus_node);
 		if (v < 0) {