Need UART "Flush" method on RP2040

For applications where using RS485 integrated circuits such as the MAX485 we need a way to know that the peripheral is done transmitting. I've checked and the RP2040 does have a BUSY bit in a regsiter, so ideally the method would be simple as

```go

func (u *UART) Flush() {
     for u.Bus.UARTFR.Get() & (1<<3) != 0 { }
}
```

See page 431 of RP2040 datasheet.