Improve uart_read_frame

[hmaerki]

This is a request to replace async def _uart_read_frame().

Problem 1: The current implementation of async def _uart_read_frame() contains two nested loops. Why? The timeout calculation does not seem to match the modus-spec.
Problem 2: Line await asyncio.sleep_ms(self._inter_frame_delay // 10) # 175 ms, arbitrary for now.
_inter_frame_delay is in micro seconds, hoever used in sleep_ms. Therefore, the timeout is way too long

My proposal uses only 1 loop: The frame is finished, when after the last character which was successfully read there is silence of 1.5 characters (or 4.5 characters).

I tested the following code on a RP2 measuring the times with the scope.

[GimmickNG]

@hmaerki Thanks for the patch! I hadn't worked with Modbus RTU directly so my earlier code was rather arbitrary based on the results of @beyonlo's testing. This should now hopefully be more robust :)