V4L/DVB (11251): tuner: prevent invalid initialization of t->config in set_type

Drivers that don't set "config" directly in the set_type function will
end up with an invalid configuration value. Check that the value is sane,
otherwise initialize to 0.

Thanks to James Edward Geiger & Steven Toth for reporting this bug.

Cc: Steven Toth <stoth@linuxtv.org>
Cc: James Edward Geiger <james.e.geiger@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>