HID: fix reversed logic in disconnect testing of hiddev

The logic for testing for disconnection is reversed in an ioctl leading
to false reports of disconnection.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Tested-by: Folkert van Heusden <folkert@vanheusden.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>