Input: serio_raw - really signal HUP upon disconnect

Commit 8c1c10d5 attempted to signal
POLLHUP | POLLERR condition when polling disconnected device,
unfortunately it did not do it quite correctly.

Reported-by: Che-Liang Chiou <clchiou@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>