Return the callback method result from check_msg()

[e-tobi]

I find it useful to return the result of the callback method from check_msg(). This way the callback can process the MQTT message and pass some information up to the main loop calling check_msg().

Things I haven't changed in the PR:

• It might also make sense to return None instead of the control field value in https://github.com/fizista/micropython-umqtt.simple2/blob/35daf4d273edc98a51cf29547603143ed6adcc20/src/umqtt/simple2.py#L461 if the message type is not expected.
• I've not touched the minimized version - not sure how this gets created

[fizista]

Unfortunately, this is a change to the application API. Some programs will stop working because of this change.