fixing return value of python bindings for cudafeatures2d methods

[apatsekin]

Related bug: opencv/opencv#19890
Tested changes locally on Ubuntu 18.4. Works as expected.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• [V] I agree to contribute to the project under Apache 2 License.
• [V] To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• [V] The PR is proposed to proper branch
• [V] There is reference to original bug report and related work
• [V] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• [V] The feature is well documented and sample code can be built with the project CMake

force_builders=Custom
buildworker:Custom=linux-4,linux-6
build_image:Custom=ubuntu-cuda:18.04

[apatsekin]

not sure I have access rights to rerun CN Jenkins builds.