Bluetooth: controller: LLL: Add peek extra_data related with lll_adv_sync

Configuration of CTE for transmission is stored in extra_date member
of a lll_adv_sync object. PDUs in periodic advertising chain share the
same CTE configuration, so there is single instance of the extra_data.
To configure CTE transmission for every PDU in periodic advertising
chain a new function was introduced to peek extra_data instance without
peeking new PDU.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>