Remove powerDown() from write command.

Remove powerDown() from write command. Now, we can use Power On/Off from user application. Necessary because XTAL startup time is not the same for every NRF board.PowerUp can take more than 1 ms (see 6.1.7 "Tpd2stdby" in nRF24L01+ product specification). For my test on Olimex MOD-24LR module delayMicroseconds(150) after PowerUp on startWrite is too short and write don't work. It's difficult to have an optimized value for every case. So, it's probably better to do this on user app.