Bluetooth: controller: minor refactoring in nrf5 radio driver

This commit moves the radio enable on-tick functionality in
function hal_radio_enable_on_tick_ppi_config_and_enable() in
radio_nrf5_ppi.h.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>