[media] em28xx: fix snapshot button support

The snapshot button support is currently broken, because module em28xx-rc is
loaded only if the device has remote control support.
Fix it by also loading this module if the device has a snapshot button.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>