drivers: mdio: fix incorrect return usage in `mdio_bus_enable/disable`

Remove incorrect return statements in the `z_impl_mdio_bus_enable` and
`z_impl_mdio_bus_disable` functions within the MDIO driver API.
These functions are intended to call the `bus_enable` and `bus_disable`
methods of the MDIO driver API without returning a value,
as they are defined to return void.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>