Fix SerialUART flush (#215)

Serial1.flush/Serial2.flush was not waiting for the proper FIFO to clear.
Use the proper call from the Pico SDK.

Thanks to Peter Remias for noting it.

cores/rp2040/SerialUART.cpp

@@ -161,7 +161,7 @@ void SerialUART::flush() {
     if (!_running || !m) {
         return;
     }
-    uart_default_tx_wait_blocking();
+    uart_tx_wait_blocking(_uart);
 }
 
 size_t SerialUART::write(uint8_t c) {