[media] cxd2820r: Update frontend capabilities to advertise QAM-256

This is supported in DVB-T2 mode, so added to the T/T2 frontend.

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