net-next: mediatek: use mdiobus_free() in favour of kfree()

The driver currently uses kfree() to clear the mii_bus. This is not the
correct way to clear the memory and mdiobus_free() should be used instead.
This patch fixes the two instances where this happens in the driver.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>