Input: db9 - 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/input/joystick/db9.c: In function ‘db9_saturn_read_packet’:
drivers/input/joystick/db9.c:256:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (tmp == 0xff) {
      ^
drivers/input/joystick/db9.c:263:2: note: here
  default:
  ^~~~~~~

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

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: Dmitry Torokhov <dmitry.torokhov@gmail.com>