Flush implementation differ to pyserial

[Hadatko]

Hello, is this flush implementation ok? Wouldn't be better to implement to match pyserial documentation?

https://github.com/pyserial/pyserial-asyncio/blob/44d470181af989fb35fbe5062f47f8eaf5b7939a/serial_asyncio/init.py#L194

[rob-smallshire]

I agree that the current implementation doesn't look ideal.