[media] cxd2841er: force 8MHz bandwidth for DVB-C if specified bw not supported

if specified DVB-C bandwidth not supported then force 8MHz.
Should work for most cases.

Signed-off-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>