STM32 Fix i2c_compute_timing() API

[pennam]

Summary of changes

#14557 has introduced an issue that was already raised here #14030 (comment). The fix is the same.

Impact of changes

STM32 with timing algo enabled

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

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

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Reviewers

@jeromecoutant @affrinpinhero-2356 @facchinm

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[ciarmcom]

@pennam, thank you for your changes.
@jeromecoutant @ARMmbed/mbed-os-maintainers please review.

[mbed-ci]

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jenkins-ci/mbed-os-ci_unittests	✔️
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jenkins-ci/mbed-os-ci_cmake-example-ARM	✔️
jenkins-ci/mbed-os-ci_build-example-ARM	✔️
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jenkins-ci/mbed-os-ci_build-example-GCC_ARM	✔️
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