[media] media drivers: annotate fall-through

Avoid warnings like those:

drivers/media/pci/ddbridge/ddbridge-core.c: In function 'dvb_input_detach':
drivers/media/pci/ddbridge/ddbridge-core.c:787:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (input->fe) {
      ^
drivers/media/pci/ddbridge/ddbridge-core.c:792:2: note: here
  case 4:
  ^~~~
...

On several cases, it is just that gcc 7.1 is not capable of
understanding the comment, but on other places, we need an
annotation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/dvb-frontends/af9013.c

@@ -535,6 +535,7 @@ static void af9013_statistics_work(struct work_struct *work)
 	switch (state->statistics_step) {
 	default:
 		state->statistics_step = 0;
+		/* fall-through */
 	case 0:
 		af9013_statistics_signal_strength(&state->fe);
 		state->statistics_step++;