STM32 EMAC : remove TIMEOUT issue when cable is not plugged yet

[jeromecoutant]

Summary of changes

This fixes #12577

ST Ethernet river implementation is not very well implemented...
Init procedure needs Ethernet cable to be plugged in order to return OK!
Workaround is then to not return this error case to higher layer in this case.

@LMESTM
@JojoS62
@artokin

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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[JojoS62]

I can test this soon.

[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
Build artifacts