Detach pins in HardwareSerial::setPins()

[SuGlider]

Description of Change

There is a bug in HardwareSerial::setPins() that sets the UART pins internally to the wrong value at the end of the operation, which may lead to errors in HardwareSerial::end(), not dettaching the right GPIO from UART.

This PR also makes sure that sequential HardwareSerial::setPins() will always detach previous attached GPIOs.

Tests scenarios

// Check previous GPIOs with a Logic Analyzer
void setup() {
  Serial.begin(115200);  // sets default RX and TX
  Serial.println("This will go out in the default TX pin");
  Serial.flush();
  
  Serial.setPins(-1, 2);  // Now TX shall be GPIO2 and default TX GPIO can't send any data
  Serial.println("This line can't be seen in the default TX GPIO!");
  Serial.flush();
}

void loop(){
}

Related links

Closes #8607

[SuGlider]

@me-no-dev - I had to create a new PR in order to rebase it to master branch.

[SuGlider]

@me-no-dev - This PR will remain as draft. Please review and merge #8629 to 2.0.13