Revert powerUp() delay to 5ms delay

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
the CEis set high.

5ms is the maximum defined in the v1.0 datasheet