drivers: clock_control: nrf: Add missing flags clearing

When BT dedicated API was turning off the HF clock it was not resetting
status flags. When onoff API was attempting to request HF clock after
that it was detecting unexpected state as status flag was indicating
as if HF clock was on.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>