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>