The copy_to_user() function returns the number of bytes which it wasn't
able to copy but we want to return a negative error code.

Fixes: 9d1a546c ("docs: Sample driver to demonstrate how to use Mediated device framework.")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: NKirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>