V4L/DVB (9433): Limit bandwidth with a 3dB response fall

In this case, we will be sure that there exists a carrier
and the center frequency would be simply f1 - f2. In such
a circumstance, the TR loop is simplified to doing a simple
check for timing, rather than searching for the same.

Signed-off-by: Arvo Jarve <arvo@softshark.ee>
Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>