[media] ts2020: improve filter limit calc

* We don't need calculate channel bandwidth from symbol rate as it
is calculated by DVB core.

* Use clamp() to force upper/lower limit of filter 3dB frequency.
Upper limit should never exceeded 40MHz (80MHz BW) in any case,
though...

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>