[media] cxd2820r: wait demod lock for DVB-C too

Fix yet another bug introduced be recent cxd2820r multi-frontend to
single-frontend change.

Finally, we have at least almost working picture for DVB-C too.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>