[media] em28xx: Show a warning if the board does not support remote controls

This simply shows a little warning if the board does not have remote
control support. This should make it easier for users to see if they
have misconfigured their system or if the driver simply does not have
rc-support for their card (yet).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>