net: mdio-gpio: Allow for unspecified bus id

When the bus id was supplied via a struct platform_device, the driver wasn't
handling -1 to mean an unspecified id of the only instance of this driver,
as the platform spec requires.
Signed-off-by: Bert Vermeulen <bert@biot.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/mdio-gpio.c

@@ -168,7 +168,10 @@ static struct mii_bus *mdio_gpio_bus_init(struct device *dev,
 		if (!new_bus->irq[i])
 			new_bus->irq[i] = PHY_POLL;
 
+	if (bus_id != -1)
 		snprintf(new_bus->id, MII_BUS_ID_SIZE, "gpio-%x", bus_id);
+	else
+		strncpy(new_bus->id, "gpio", MII_BUS_ID_SIZE);
 
 	if (devm_gpio_request(dev, bitbang->mdc, "mdc"))
 		goto out_free_bus;