media: mxl5xx: add a fall-trough annotation

As pointed by Daniel Scheller <d.scheller@gmx.net>, sparse
warns that a segment may fall through. We're deprecating
this warning in favor of gcc 8 equivalent warning.

In this specific case, the code for DVB-S applies for DVB-S2.
In a matter of fact, several things there (like rolloff
factor logic) assume that the transponder is DVB-S2.

So, add the corresponding fall-trough markup, for gcc to
properly ignore it.

Reported-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>