Input: serio_raw - explicitly mark disconnected ports as dead

Instead of relying on setting serio_raw->serio to NULL upon disconnecting
ports mark them explicitly as "dead". Also take and carry reference to
underlying serio port to make sure it does not go away until we are done
with it.

Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>