Input: libps2 - 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/serio/libps2.c: In function ‘ps2_handle_ack’:
drivers/input/serio/libps2.c:407:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (ps2dev->flags & PS2_FLAG_NAK) {
      ^
drivers/input/serio/libps2.c:417:2: note: here
  case 0x00:
  ^~~~

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