ATHandler build warning fixed

[AnttiKauppila]

Description (required)

Build time warning fixed for ATHandler

Summary of change (What the change is for and why)

Documentation (Details of any document updates required)

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Pull request type (required)

[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 (required)

[] 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 (optional)

@mirelachirica

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Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[ciarmcom]

@AnttiKauppila, thank you for your changes.
@mirelachirica @ARMmbed/mbed-os-wan @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts