[media] tuner-core: fix a 2.6.39 regression with mt20xx

As Simon reported, digital TV broke with mt20xx tuner due to
commit ad020dc2.

The mt20xx tuner passes V4L2_TUNER_DIGITAL_TV to tuner core. However, the
check_mode code now doesn't handle it well. Change the logic there to
avoid the breakage, and fix a test for analog-only at g_tuner.

Reported-by: Simon Arlott <simon@fire.lp0.eu>
Tested-by: Simon Arlott <simon@fire.lp0.eu>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>