If serio_raw_write was always returning number of bytes successfully
sent to serio port and never signalled error condition to the caller.
Change it so that for completely failed transfers appropriate error
code returned to the caller (partially successful writes still return
number of bytes transferred).
Reported-by: Che-liang Chiou <clchiou@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>