bluetooth: controller: nrf5: Reorder used PPI channels

The channels assigned to the controller are reordered so that the ones
previously used and now available for other purposes have continuous
numbers. When the controller can take advantage of the pre-programmed
PPI channels (when TIMER0 is used as the event timer), the now free
channels are 0-4, when it cannot, it is the channel 0.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>