Fixing 300 baud communication for serial.

Because UBBR is only 12 bits, we were overflowing it at 300 baud because of the use of the U2X bit.  Now we turn off U2X if it would yield a UBBR value that would overflow.

Note that this breaks 300 baud communication with the computer on the Uno and Mega 2560 because the 8U2 USB-serial firmware has this same bug (and previously they cancelled each other out).  Since, however, it seems more likely that people will need to use 300 baud to communicate with other (legacy) hardware than with the computer, I'm making this change.  Issue for 8U2 firmware bug: http://code.google.com/p/arduino/issues/detail?id=542

http://code.google.com/p/arduino/issues/detail?id=522

hardware/arduino/cores/arduino/HardwareSerial.cpp

@@ -204,6 +204,8 @@ void HardwareSerial::begin(unsigned long baud)
     use_u2x = false;
   }
 #endif
+
+try_again:
 
   if (use_u2x) {
     *_ucsra = 1 << _u2x;
@@ -212,6 +214,12 @@ void HardwareSerial::begin(unsigned long baud)
     *_ucsra = 0;
     baud_setting = (F_CPU / 8 / baud - 1) / 2;
   }
+
+  if ((baud_setting > 4095) && use_u2x)
+  {
+    use_u2x = false;
+    goto try_again;
+  }
 
   // assign the baud_setting, a.k.a. ubbr (USART Baud Rate Register)
   *_ubrrh = baud_setting >> 8;