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>

drivers/media/dvb-frontends/mxl5xx.c

@@ -739,6 +739,7 @@ static int get_frontend(struct dvb_frontend *fe,
 		default:
 			break;
 		}
+		/* Fall through */
 	case SYS_DVBS:
 		switch ((enum MXL_HYDRA_MODULATION_E)
 			reg_data[DMD_MODULATION_SCHEME_ADDR]) {