receive(): don't touch FIFO on timeout

[bablokb]

This fix dramatically reduces lost packages when polling for packets.

Consider this use case:

while True:
  p = my_rfm95x.receive(keep_listening=True, timeout=2)
  if p:
   do_something(p)

If a packet arrives partially just before the timeout kicks in, the packet is lost because the receive-method switches to idle, clears the queue and the interrupt. This patch does not do and just returns, so the next iteration will pick up the packet.

Note that the problem is independent of the actual value of timeout, because with any timeout-value you can run into this problem. The problem is more prominent with low bitrates, since transfers take much longer and therefore the chance of hitting the timeout is much higher.

The new combined library has the same problem I think, but I did not test because my application still uses this library.

[jerryneedell]

@bablokb Thank you for this. It looks like an excellent improvement. I have been troubled by this for awhile and just never thought of such a simple solution!
Unfortunately, I have very limited internet access today ( my provider is doing "upgrades"). When I can get back online fully. I'll run some tests and will also implement this for the combined RFM library.

[jerryneedell]

This looks good to me - thank you!

[jerryneedell]

@bablokb Thank you for this -- I have also implemented it for the adafruit_rfm69 and adafruit_rfm libraries.
adafruit/Adafruit_CircuitPython_RFM69#58
adafruit/Adafruit_CircuitPython_RFM#21