Input: ps2-gpio - flush TX work when closing port

To ensure that TX work is not running after serio port has been torn down,
let's flush it when closing the port.

Reported-by: Sven Van Asbroeck <thesven73@gmail.com>
Acked-by: Danilo Krummrich <danilokrummrich@dk-develop.de>
Reviewed-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>