auxdisplay: panel: Change comments to silence fallthrough warnings

Compiling with W=1 with gcc 7.2.0 gives 3 warnings like:

  drivers/auxdisplay/panel.c: In function ‘panel_process_inputs’:
  drivers/auxdisplay/panel.c:1374:17: warning: this statement may fall
  through [-Wimplicit-fallthrough=]

Cc: Willy Tarreau <w@1wt.eu>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>