fix high speed communication issue

under high speed communication(tested with 2Mbps), isDataregisterEmptyUART() gives false true and return before the completion of transmission.

Author: ROBOTIS-Will

cores/arduino/SERCOM.cpp

@@ -112,8 +112,8 @@ void SERCOM::enableUART()
 void SERCOM::flushUART()
 {
   // Skip checking transmission completion if data register is empty
-  if(isDataRegisterEmptyUART())
-    return;
+//   if(isDataRegisterEmptyUART())
+//     return;
 
   // Wait for transmission to complete
   while(!sercom->USART.INTFLAG.bit.TXC);