net: ethernet: ixgbe: don't call devm_mdiobus_free()

The idea behind devres is that the release callbacks are called if
probe fails. As we now check the return value of ixgbe_mii_bus_init(),
we can drop the call devm_mdiobus_free() in error path as the release
callback will be called automatically.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>