Fix bug in baudrate compensation. (#44)

* Fix bug in baudrate compensation. The code is now in accordance to the
data sheet. The previous code would in effect either apply twice the
compensation, or apply no compensation, depending on the polarity of
the compensation.

* Improve readability of expression for baudrate compensation.
It is now exactly the same as in the documentation.
The net result is the same as the previous version.

* Since we have code to correct for errors in F_CPU to generate
the right baudrate, then we should use F_CPU, the specified CPU
frequency as basis, not the F_CPU with whatever corrections.

Author: egilkv

cores/arduino/UART.cpp

@@ -154,16 +154,15 @@ void UartClass::begin(unsigned long baud, uint16_t config)
     uint8_t oldSREG = SREG;
     cli();
 
-    baud_setting = (((8 * F_CPU_CORRECTED) / baud) + 1) / 2;
+    baud_setting = (((8 * F_CPU) / baud) + 1) / 2;
     // Disable CLK2X
     (*_hwserial_module).CTRLB &= (~USART_RXMODE_CLK2X_gc);
     (*_hwserial_module).CTRLB |= USART_RXMODE_NORMAL_gc;
 
     _written = false;
 
     int8_t sigrow_val = SIGROW.OSC16ERR5V;
-    baud_setting *= (1024 + sigrow_val);
-    baud_setting /= (1024 - abs(sigrow_val));
+    baud_setting += (baud_setting * sigrow_val) / 1024;
 
     // assign the baud_setting, a.k.a. BAUD (USART Baud Rate Register)
     (*_hwserial_module).BAUD = (int16_t) baud_setting;