Add powerUp to begin(), increase powerUp delay

- Found I could create a hang condition on startup with some code.
a: Now power up the radio on begin() to simplify things
b: Increase delay to 5ms for powerUp, although it should only take 2ms
tops, this prevents the hanging condition.

RF24.h

@@ -996,6 +996,7 @@ private:
  * - Changes to read() functionality have increased reliability and response
  * - Extended timeout periods have been added to aid in noisy or otherwise unreliable environments
  * - Delays have been removed where possible to ensure maximum efficiency
+ * - Untested: Arduino Due and ATTiny 84/85 support: Do NOT #include <SPI.h> with ATTiny.
  * - More! See the links below and class documentation for more info.
  *
  * If issues are discovered with the documentation, please report them here: <a href="https://github.com/TMRh20/tmrh20.github.io/issues"> here</a>
@@ -1025,5 +1026,5 @@ private:
  */
 
 #endif // __RF24_H__
-// vim:ai:cin:sts=2 sw=2 ft=cpp
+