rf: JIT scheduler fixes and improvements

- Prevent double callback with ADV re-scheduling
  For configurations using CONFIG_BT_CTLR_JIT_SCHEDULING, when last
  ADV before initiating connection is re-scheduled with a small delay,
  the disabled_cb would be called a second time. This would unexpectedly
  invoke a second conn setup with illegal parameters.
  To avoid this JIT scheduler phenomenon, clear the ADV disabled_cb
  when invoked.
- Add priority to LLL header. This allows the conn priority to be
  associated with the object. Used in vendor LLL implementation.

Signed-off-by: Morten Priess <mtpr@oticon.com>