Reliability improvements for RPi

- In testing with python wrapper, found setting CE(HIGH) sooner in
startListening() prevents issues
- Changed delay between available() checking on RPi to prevent what
seems like saturation of the RF24 module with SPI requests
- Reduces errors when switching between RX/TX modes
Remove unnecessary delay

Further testing indicates this is not needed
Revert last commit, if defined... delay

Delay is necessary for faster devices in some cases
- reduced delay by 30us for testing further