STM32: fix HRTIM pwm init and corner case

[leonardocavagnis]

Summary of changes

Fixing HRTIM (High Resolution Timer) PWM initialization procedure that causes erroneous initialization of the duty cycle of the HRTIM channels in STM32 microcontrollers.

Impact of changes

Migration actions required

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)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

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Reviewers

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