Input: i8042 - fix shutdown issue on some boxes with active MUXes

MUX error handling has a workaround for KBCs that get confused which
port data came from and signal MUXERR while data is actually good.
Unfortunately this workaround hurts with KBCs that signal timeouts
as 0xfc (spec says that only 0xfd, 0xfe and 0xff are alowed with
MUXERR) since it causes endless attempts to rescan i8042 serio
ports. The solution is to treat 0xfc as timeout (0xfe).

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>