STM32: Update and align serial_clear implementations

[LMESTM]

Description

Clear RXNE flag by reading the RX register and align this implementation on all families.

Summary of change

This solves the issue reported here: #11893

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

There is no known tests for serial_clear API. 
Also there is no clear documentation of what behavior is actually expected from serial_clear. This implementation is based on current understanding.

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Reviewers

@ARMmbed/team-st-mcd

[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 12 of 12 test jobs passed
Build number : 1
Build artifacts

[0xc0170]

@ARMmbed/team-st-mcd Any review, looks like this is ready almost