[media] media: platform: s5p: Constify platform_device_id

The platform_device_id is not modified by the driver and core uses it as
const.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
[k.debski@samsung.com: Fix misspelling in title (s/sp5/s5p/)]
Signed-off-by: Kamil Debski <k.debski@samsung.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>