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

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

As Simon reported, digital TV broke with mt20xx tuner due to
commit ad020dc2fe9039628cf6cef42cd1b76531ee8411.

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 <[email protected]>
Tested-by: Simon Arlott <[email protected]>
Cc: [email protected]
Signed-off-by: Mauro Carvalho Chehab <[email protected]>