Fix inconsistent function signature in CDC.cpp

USBAPI.h declares the return type of Serial_::baud() as `uint32_t`

Author: weliveindetail

cores/arduino/USB/CDC.cpp

@@ -336,7 +336,7 @@ int32_t Serial_::readBreak() {
 	return ret;
 }
 
-unsigned long Serial_::baud() {
+uint32_t Serial_::baud() {
 	return _usbLineInfo.dwDTERate;
 }