wrong return value for read timeout in Arduino client

[ppescher]

The return value rc is initialized to -1, which is the value returned when nothing has been received in the specified time frame:
https://github.com/eclipse/paho.mqtt.embedded-c/blob/29ab2aa29c5e47794284376d7f8386cfd54c3eed/MQTTClient/src/arduino/IPStack.h#L48

It may appear to work with faster network links because received data is available quickly, before the client has the chance to check. But it can still fail and it does with slower links or slower server side.

Suggest to replace the above initialization code with rc=0.