Fix Unbuffered serial enable/disable issue on Cypress kits

[praveenCY]

Summary of changes

Fix for UART init and deinit in cypress kits using UnbufferedSerial enable_input/enable_output functions.

Problem: Calling below sequence for UnbufferedSerial causes UART logs to be stopped.

1. pc.enable_input(false);
2. pc.enable_output(false);
3. pc.enable_input(true);
4. pc.enable_output(true);

Reason: serial_free is called during step 2 and serial_init called during step 3 doesnot really call Cypress uart init function due to be stdio_uart_inited check.

Solution: Clear stdio_uart_inited during serial_free so that serial init will properly call cyhal_uart_init

Impact of changes

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

[] 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

@ARMmbed/team-cypress

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[ciarmcom]

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

[0xc0170]

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[mbed-ci]

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