media: dvb_frontend: add missing DSS switch cases

While the documentation mentions the delivery system DSS as a satellite
system, it was missing from all but one switch statement in the DVB
frontend code, leading to tuning failures, because the frequency was not
correctly handled as being in kHz rather than Hz.

Add the missing switch cases so that DSS is handled like the other
satellite systems. For the rolloff, assume 0.20 as per one publication
found via Internet search.

Link: https://lore.kernel.org/linux-media/trinity-5f5afda9-657a-4a91-bf15-842f4f249535-1641958421391@3c-app-gmx-bap21


Signed-off-by: Robert Schlabbach <robert_s@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>