Send SIGIO/POLL_HUP and otherwise wake up waiters when corresponding serio
port is being disconnected. Also check if port is dead in serio_raw_poll
and signal POLLHUP|POLLERR.

This should speed up process of releasing dead devices by userspace
applications.
Reviewed-by: NWanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>