Bluetooth: controller: split: Fix Low Latency Prepare Done

Fix the Low Latency Prepare Done that disables ULL_LOW
context executions inside a radio event, so that
ticker_job_idle_get is invoked from ULL_LOW instead of LLL
context so that its enqueued alongwith others scheduled
in ULL_LOW. This is avoid premature disable of ULL_LOW
context executions.

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