Fix DevAddr filter based on NetID configuration

[johanstokking]

Summary

Fixes the DevAddr that PBA uses for subscribing as home network when there's only a NetID configured.

Changes

• The prefix length is now based on the NwkAddr bits instead of the NwkID bits.

Testing

None, we don't have unit testing for this. The code is taken from Network Server.

Regressions

None, this is a bugfix.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• The steps/process to test this feature are clearly explained including testing for regressions.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.