[feature-wisun] Added missing const qualifier to mesh ethernet mac addr get

[mikaleppanen]

Summary of changes

Added missing const qualifier to mesh ethernet mac addr get. This corrects coverity warning.

This is feature-wisun version of: #14261

Impact of changes

None

Migration actions required

None

Documentation

None

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@mikter @artokin

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[ciarmcom]

@mikaleppanen, thank you for your changes.
@artokin @mikter @ARMmbed/mbed-os-maintainers please review.

[ciarmcom]

This pull request has automatically been marked as stale because it has had no recent activity. @artokin, @mikter, @ARMmbed/mbed-os-maintainers, please complete review of the changes to move the PR forward. Thank you for your contributions.

[0xc0170]

Once we get the code freeze for Mbed OS 6.x , we should be able to run all 5.15.x jobs. sorry for the delay

[ciarmcom]

This pull request has automatically been marked as stale because it has had no recent activity. @ARMmbed/mbed-os-maintainers, please start CI to get the PR merged.

[adbridge]

CI started

[adbridge]

CI restarted

[mbed-ci]

Test run: SUCCESS

Summary: 10 of 10 test jobs passed
Build number : 1
Build artifacts