Bluetooth: controller: nRF5x: Fix lll LF clock wait

Fix lll_clock_wait function to wait for LF clock to settle
only once after power up.

Regression introduced in commit 2b476307 ("bluetooth:
controller: Adapt to onoff clock control").

Fixes #30480.

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