Sets the UART pins internally

[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

[P-R-O-C-H-Y]

@SuGlider PTAL

[SuGlider]

Closing it in favor of #8619 -- for rebasing it to master branch