Duplicate read NRF_CONFIG per #203 in powerUp()

RF24.cpp

@@ -760,7 +760,7 @@ void RF24::powerUp(void)
 
    // if not powered up then power up and wait for the radio to initialize
    if (!(cfg & _BV(PWR_UP))){
-      write_register(NRF_CONFIG,read_register(NRF_CONFIG) | _BV(PWR_UP));
+      write_register(NRF_CONFIG, cfg | _BV(PWR_UP));
 
       // For nRF24L01+ to go from power down mode to TX or RX mode it must first pass through stand-by mode.
 	  // There must be a delay of Tpd2stby (see Table 16.) after the nRF24L01+ leaves power down mode before