V4L/DVB (12686): dvb-core: check supported QAM modulations

Check the modulation in dvb_frontend_check_parameters against
frontend's capabilties for FE_QAM devices.

Signed-off-by: Janne Grunau <j@jannau.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>