Bluetooth: Controller: Fix missing DF related radio register reset

Fix missing DF related Radio register reset on radio_reset().

Regression in commit 465a9618 ("Bluetooth: Controller:
Explicitly set all bits of used radio registers") due to the
removal of NRF_RADIO->POWER register use that was used to
reset all Radio registers on every new Radio Event.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>