Fix for issue #12268 (SerialBase.cpp: fix initialization list)

[mprse]

Summary of changes

This PR fixes issue #12268.
This PR changes the order of the initialization list in SerialBase.cpp. This should fix the compilation warnings.

Snippet from the build which proves that there are no warnings while compiling SerialBase.cpp with this patch:

Compile [  7.4%]: Ticker.cpp
Compile [  7.5%]: SerialBase.cpp
Compile [  7.6%]: Timer.cpp
Compile [  7.8%]: TimerEvent.cpp

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)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

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Reviewers

@JanneKiiskila

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[ciarmcom]

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

[JanneKiiskila]

LGTM. Thanks!

[adbridge]

@mprse what is the PR type? Please fill this in

[adbridge]

Also when fixing issues, please provide a brief summary of what the issue is and how it has been fixed in the header (rather than just referencing the Issue number).

[adbridge]

CI started

[mbed-ci]

Test run: SUCCESS

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