[media] ngene: separate demodulator and tuner attach

Previously, demodulator and tuner attach was done in the
demod_attach callback. Migrate the tuner part in the
tuner_attach callback in ngene_info to do thing in right place.

Signed-off-by: Patrice Chotard <patricechotard@free.fr>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>