Return packet type processed in process()

[madleech]

The readme specifically says for process():

Return value is NULL for no valid packet received, or one of CMRI::INIT, CMRI::SET, CMRI::POLL depending on the packet type received.

However in reality it's just returning true / false. For some users, having it return the processed packet type is a useful enhancement. Lets update it to match the documentation.

Note: although this is a behaviour change, anyone doing if (cmri.process()) will see unchanged behaviour, as false and NOOP are both 0 and so equivalent.