staging: comedi: ni_660x: fix memory barrier without comment

Fix the checkpatch.pl issue. Move the memory barrier to a better place.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>