[media] media: Initialize the media core with subsys_initcall()

Media-related drivers living outside drivers/media/ (such as the UVC
gadget driver in drivers/usb/gadget/) rely on the media core being
initialized before they're probed. As drivers/usb/ is linked before
drivers/media/, this is currently not the case and will lead to crashes
if the drivers are not compiled as modules.

Register media_devnode_init() as a subsys_initcall() instead of
module_init() to fix this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Bhupesh Sharma <bhupesh.sharma@st.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Tested-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>