submodule: bump openthread from `5cfbcfc` to `dff9111` (#297)
submodule: bump openthread from `5cfbcfc` to `dff9111` (#297)

Bumps [openthread](https://github.com/openthread/openthread) from `5cfbcfc` to `dff9111`.
- [Commits](https://github.com/openthread/openthread/compare/5cfbcfcbf4cbf1e33cdeb00db9b7f8903374c4e2...dff9111d0c5898b76c5ed429bccd8aa1cb3dd023)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
openthread/ot-samr21,openthread/ot-samr21