Fix incorrect TWBR register in twi1.c

Closes #244

Author: MCUdude

avr/libraries/Wire1/src/utility/twi1.c

@@ -83,7 +83,7 @@ void twi_init1(void)
 
   /* twi bit rate formula from atmega128 manual pg 204
   SCL Frequency = CPU Clock Frequency / (16 + (2 * TWBR))
-  note: TWBR should be 10 or higher for master mode
+  note: TWBR1 should be 10 or higher for master mode
   It is 72 for a 16mhz Wiring board with 100kHz TWI */
 
   // enable twi module, acks, and twi interrupt
@@ -126,7 +126,7 @@ void twi_setAddress1(uint8_t address)
  */
 void twi_setFrequency1(uint32_t frequency)
 {
-  TWBR = ((F_CPU / frequency) - 16) / 2;
+  TWBR1 = ((F_CPU / frequency) - 16) / 2;
 
   /* twi bit rate formula from atmega128 manual pg 204
   SCL Frequency = CPU Clock Frequency / (16 + (2 * TWBR1))