Fix MIMXRT105x serial_format() not working

[multiplemonomials]

Summary of changes

This applies the patch found in this issue: ARMmbed#15342 . The code for MIMRT serial_format() was obviously broken as it is using a uint8_t to store the 32-bit CTRL register:

Impact of changes

serial_format() was obviously broken on MIMXRT105x targets.

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

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

Was tested by @grumpyengineer in their issue.

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Reviewers

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