FPGA SPI : decrease test frequency for ASYNC test cases

[jeromecoutant]

Summary of changes

With NUCLEO_WB55RG target, SPI async use test cases are sometimes OK, sometimes not...
Issue is due to the low frequency of this target (32MHz)

Proposition is to decrease the test frequency to allow the maximum of targets to be tested.

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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[ciarmcom]

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

[adbridge]

CI started

[mbed-ci]

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