[feature-wisun] Make _unacknowledged_ticks uint32_t to prevent overflow

[mikaleppanen]

Summary of changes

Changes _unacknowledged_ticks from 8 to 32 bit to prevent overflow when interrupts are not being handled for a while.
This could happen due to, for example, debugging or a long critical section.

Fixes #13801
Fixes #13772

This is feature_wisun version of the pull request: #13867

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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[mikaleppanen]

feature-wisun fix for the systick issue @amarjeet-arm @teetak01 @TuomoHautamaki

[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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