Fix mesh connect semaphore not releasing causing blockage

[YannCharbon]

Summary of changes

When MeshInterface::disconnect() is called after MeshInterface::connect() has been called but before the Wi-SUN connection process has ended, there is a deadlock situation which prevents restarting the connection process. This is due to the fact that the connection semaphore is only released when the interface reaches a "CONNECTED" state.

This PR fixes this deadlock by releasing the connection semaphore if MeshInterface::disconnect() is invoked.

Impact of changes

Migration actions required

Documentation

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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

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[YannCharbon]

@0xc0170 I recreated this PR and #15366 because I had some troubles on my side. I took the opportunity to fix the dependency issue between the two PRs, so everything should be alright now. Excuse me for the inconvenience.

[0xc0170]

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[mbed-ci]

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