STM serial init: Set pin function only if pin is defined (not NC)

[mprse]

Summary of changes

This is patch for issue #12172.

During serial initialization set pin function only if the pin is specified (not NC).

Verified that the following code works with the patch:

main()
{
 Serial tx_only_serial(PE_8, NC); // halts here
 tx_only_serial.putc('A');
}

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

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[mbed-ci]

Test run: FAILED

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

Failed test jobs:

• jenkins-ci/mbed-os-ci_greentea-test

[adbridge]

Looks like that failure may have been a device issue in the ci. Restarting.

[mbed-ci]

Test run: FAILED

Summary: 1 of 11 test jobs failed
Build number : 2
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_greentea-test

[0xc0170]

@mprse There are IOERR_SERIAL for one disco board, are these related?

[mprse]

I saw the results. Tried to reproduce this locally, but without success. Have you seen such failures before? This PR changes the STM UART driver a bit, but not sure if these failures are related.

[0xc0170]

Restarted the tests. I haven't seen ioserial today but the target previously had sometimes serial issue (if this is confirmed, we should check what board is it, might need hardware restart).

[0xc0170]

It was just a target serial error. I'll monitor the next tests after this integration just in case.