The change simplifies dereferences to the mediated struct device, also
it allows to limit the scope of the platform device usage to probe and
remove functions only.

Non-functional change.
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>