Bluetooth: Controller: Fix LLL events pending check from ULL Low

ULL reference count is checked in ULL_LOW context to decide
if LLL events are pending, but the reference count can be
decremented by the ULL HIGH execution context which can
prevent the set `disabled_cb` function not being called due
to no pending event to produce the done events.

Fixed by checking the reference count in the ULL HIGH
execution context using a mayfly to schedule the check.

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