feature_discovery: Close device before exit

Close the device fd before we check for leaks during the atexit
handlers.

Signed-off-by: Chris Wilson <chris.p.wilson@intel.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>