net: mdio: mscc-miim: add local dev variable to cleanup probe function

Create a local device *dev in order to not dereference the platform_device
several times throughout the probe function.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>