[media] stv0367: signedness bug in stv0367_get_tuner_freq()

We use err to store negative error codes so it should be signed.  And
if we return an error from stv0367_get_tuner_freq() that needs to be
handled properly as well.  (param->frequency is a u32).

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>