Bluetooth: controller: Fix RL index check

In the ll_rl_pdu_adv_update() function, the check to verify if we are
dealing with an item from the resolving list or else with a simple
standard non-privacy enabled device was left over from the previous
iteration, which used negative values. Replace that check with the
proper current one, using the size of the rl array as an indicator of
whether the index is valid.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>