platforms/nuttx: Serial change fsync to tcdrain

Calling serial::write() in quick succession was blowing away the previous buffer, fsync does not guarantee that data is transmitted on serial lines. On the other hand tcdrain waits until the output buffer is empty.

platforms/nuttx/src/px4/common/SerialImpl.cpp

@@ -339,7 +339,7 @@ ssize_t SerialImpl::write(const void *buffer, size_t buffer_size)
 	}
 
 	int written = ::write(_serial_fd, buffer, buffer_size);
-	::fsync(_serial_fd);
+	tcdrain(_serial_fd); // Wait until all output is transmitted
 
 	if (written < 0) {
 		PX4_ERR("%s write error %d", _port, written);