Bluetooth: Controller: Fix repeated per sync drift compensations

Fix repeated periodic sync drift compensation invoked when
receiving chain PDUs which caused memory corruptions and
bus faults.

Use `is_aux_sched` flag in Periodic Sync's LLL context to
differentiate between the first AUX_SYNC_IND PDU followed by
use of LLL scheduling to receive following AUX_CHAIN_IND PDU
versus ULL scheduling being used to receive AUX_CHAIN_IND
PDUs.

Drift compensation to be done only using the AUX_SYNC_IND
PDU and not on reception of AUX_CHAIN_IND PDU using ULL
scheduling.

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