bluetooth: controller: nrf5: Use pre-programmed PPI channels

Several PPI channels in nRF5 family SoCs are pre-programmed with fixed
settings. A few of them can be used in the bluetooth controller instead
of the freely programmable ones that are used currently. This commit
makes such replacements where possible so that the universal channels
can be left available for other purposes.

This commit also removes macros used previously in calls to functions
enabling and disabling particular PPI channels (as it is sufficient
to use the BIT macro to set bits corresponding to the channel numbers)
to prevent such problems like the one introduces by commit
9d1ca9c3 (channel 18 changed to 17
but the related macro definition not updated).

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