fix: printer does not update TX/RX pin

[tadeubas]

What is this PR for?

Fixes an issue where the printer TX/RX pin is not updated correctly after changing the configuration.

Steps to reproduce

1. On M5StickV, go to Settings > Hardware > Printer > Driver and set it to thermal/adafruit.
   Then go to Thermal > TX Pin, set it to 39, go back, and save the settings.
2. Go to Tools > Device Tests > Print Test QR and confirm “Print as QR?”.
   (Nothing happens, as the pin is invalid.)
3. Return to Settings > Hardware > Printer > Thermal > TX Pin, set it to 35, go back, and save the settings.
4. Go again to Tools > Device Tests > Print Test QR and confirm “Print as QR?”.
   (An error occurs.)

Error

Error: Exception: [Warning] function is used by unknown (pin:39)

Result

After this PR, the error no longer occurs and the printer works correctly when following the same steps.

Changes made to:

• Code
• Tests
• Docs
• CHANGELOG

Did you build the code and tested on device?

• Yes, build and tested on M5StickV

What is the purpose of this pull request?

• Bug fix
• New feature
• Docs update
• Other

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.31%. Comparing base (86340f3) to head (c6b9991).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #810   +/-   ##
========================================
  Coverage    97.31%   97.31%           
========================================
  Files           83       83           
  Lines        10576    10576           
========================================
  Hits         10292    10292           
  Misses         284      284           

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