Timing issue if for startWrite, if not using an external clock

[fiveohhh]

In RF24::startWrite, there is a coded delay of 15us after power up. This time is in the data sheet as Tpd2stby and only valid for chips using an external clock. I'm using a Sparkfun board (https://www.sparkfun.com/products/691) and couldn't find Ls for the crystal on it, but I was getting odd behavior with the default setting of 150us. I bumped it up to 4ms and all was well.

[etemu]

Could be fixed with a dynamic (clock frequency dependent) delay.