Update serial.c for latest microchip DFP (1.6.88 from 1.1.40) (#517)

Update line 58 to make compatible with Microchip DFP 1.6.88


Co-authored-by: Joshua Yan <52796499+yanjos-dev@users.noreply.github.com>

FreeRTOS/Demo/AVR_Dx_MPLAB.X/serial/serial.c

@@ -19,10 +19,9 @@
  * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
  * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  *
- * http://www.FreeRTOS.org
- * http://aws.amazon.com/freertos
+ * https://www.FreeRTOS.org
+ * https://github.com/FreeRTOS
  *
- * 1 tab == 4 spaces!
  */
 
 
@@ -55,7 +54,7 @@ xComPortHandle xSerialPortInitMinimal( unsigned long ulWantedBaud, unsigned port
         USART1.BAUD = (uint16_t)USART_BAUD_RATE(ulWantedBaud); /* set baud rate register */
 
         USART1.CTRLA = 1 << USART_LBME_bp       /* Loop-back Mode Enable: enabled */
-                     | USART_RS485_OFF_gc       /* RS485 Mode disabled */
+                     | USART_RS485_DISABLE_gc       /* RS485 Mode disabled */
                      | 1 << USART_RXCIE_bp;     /* Receive Complete Interrupt Enable: enabled */
 
         USART1.CTRLB = 1 << USART_RXEN_bp       /* Reciever enable: enabled */