media: em28xx: annotate unchecked call to media_device_register()

Static analyzers generate alerts for an unchecked call to
`media_device_register()`. However, in this case, the device will work
reliably without the media controller API.

Add a comment above the call to prevent future unnecessary changes.

Suggested-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Fixes: 37ecc7b1

 ("[media] em28xx: add media controller support")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>