Bluetooth: Controller: nrf5_dppi.h: Use nrfx HAL to set subscriptions

A few paths used only when there is Coded Phy support were
still setting the subscribe registers directly.
Let's use the nRFx HAL for this so it works properly also
in simulation.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>