Staging: panel: Prevent double-calling of parport_release - fix oops.

This patch prevents the code from calling parport_release and
parport_unregister_device twice with the same arguments - and thus fixes an oops.

Rationale:
After the first call the parport is already released and the
handle isn't valid anymore and calling parport_release and
parport_unregister_device twice isn't a good idea.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Cc: stable <stable@kernel.org>
Acked-by: Willy Tarreau <w@1wt.eu>