Fix SPIF read dummy cycles not being reset

[boraozgen]

Summary of changes

Fixes #13428 by removing the unnecessary setting of _read_dummy_and_mode_cycles.

Citing my explanation here: #13428 (comment)

Actually the usage of _read_dummy_and_mode_cycles and _dummy_and_mode_cycles seems to be a bit inconsistent and probably needs refactoring/explanation. For this particular case, as _dummy_and_mode_cycles is set in the next line and this variable is used in the following _spi_send_read_command(), setting _read_dummy_and_mode_cycles is unnecessary. Furthermore, this causes the issue with SST26 since after _sfdp_detect_best_bus_read_mode() sets this value to 0, which is the default value for SPI chips AFAIK, the _spi_send_read_sfdp_command is called again, and it is set to 8, which is never again set back to 0.

When I remove this line, the driver works properly for both of the SPI chips I have, the SST26VF016B and W25Q32JV. I do not know the requirements for other chips and cannot test them. I can still issue a PR for this if needed. Nevertheless, I believe the whole module should be refactored with this in mind.

Impact of changes

Migration actions required

Documentation

None

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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

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Reviewers

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[ciarmcom]

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

[0xc0170]

@ARMmbed/mbed-os-core Please review

[0xc0170]

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