Bluetooth: controller: inline implementation of radio-enable-on-tIFS

This commit implements a minor refactoring of the implementation
for enabling the Radio on TIFS expiration. It inlines a function
that is defined in the local radio_nrf5_ppi header file.

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