[gen2] usart: fix D0 AF unconditionally being changed in hal_usart_end()

[avtolstoy]

Problem

Wire.begin();
Serial1.begin(115200);
Serial1.end();

The following code will leave I2C peripheral in a non-functioning state on Gen 2 platforms, because D0 Alternate-Function will be reset to default value unconditionally by a call to Serial1.end().

Steps to Test

• wiring/no_fixture UART* tests

Example App

N/A

References

• [CH70258]
• [CH70277]

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)