Fixed calculation in RFM22B::getDataRate()
Prevent buffer overflow in RFM22B::receive()
Added functions to configure CRC mode and type
Add warning about data rate setting being the TX data rate

As the data sheet Par. 3.5.7 "TX Data Rate Generator" mentions:
"The txdr register is only applicable to TX mode and does not need to be
programmed for RX mode. The RX bandwidth which is partly determined from
the data rate is programmed separately."
Merge pull request #2 from dimhoff/master

Some little bug fixes and improvements