USB: idmouse: simplify disconnect handling

Since commit d4ead16f ("USB: prevent char device open/deregister
race") core prevents further calls to open() after usb_deregister_dev()
returns so there's no need to use the interface data for
synchronisation.

This effectively reverts commit 54d2bc06

 ("USB: fix locking in
idmouse") with respect to the open-disconnect race.

Note that the driver already uses a present flag to suppress I/O post
disconnect (even if all USB I/O take place at open).

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191105103638.4929-2-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>