Make private members of HardwareSerial protected

This allows users to create subclasses.

Closes: arduino#947

Author: matthijskooijman

hardware/arduino/avr/cores/arduino/HardwareSerial.h

@@ -39,7 +39,7 @@
 
 class HardwareSerial : public Stream
 {
-  private:
+  protected:
     volatile uint8_t *_ubrrh;
     volatile uint8_t *_ubrrl;
     volatile uint8_t *_ucsra;