HID: roccat: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/hid/hid-roccat-kone.c: In function ‘kone_keep_values_up_to_date’:
drivers/hid/hid-roccat-kone.c:784:20: warning: this statement may fall through [-Wimplicit-fallthrough=]
   kone->actual_dpi = kone->profiles[event->value - 1].
   ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     startup_dpi;
     ~~~~~~~~~~~
drivers/hid/hid-roccat-kone.c:786:2: note: here
  case kone_mouse_event_osd_profile:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-roccat-kone.c

@@ -783,6 +783,7 @@ static void kone_keep_values_up_to_date(struct kone_device *kone,
 	case kone_mouse_event_switch_profile:
 		kone->actual_dpi = kone->profiles[event->value - 1].
 				startup_dpi;
+		/* fall through */
 	case kone_mouse_event_osd_profile:
 		kone->actual_profile = event->value;
 		break;