mdio_bus: Remove unneeded gpiod NULL check

The gpiod API checks for NULL descriptors, so there is no need to
duplicate the check in the driver.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>