[media] cx231xx, em28xx: pass media_device to si2157

As si2157 doesn't use the subdev, but has instead a binding
logic that doesn't have any core framework, we need to manually
pass the media_device struct via platform data on every place
it is called.

This fixes support for HVR-955Q when MC is enabled.

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