Bluetooth: ll_sync_create_cancel fails with BT_HCI_ERR_CMD_DISALLOWED before BT_HCI_EVT_LE_PER_ADV_SYNC_ESTABLISHED is generated

[ppryga-nordic]

Describe the bug
After change of moment when BT_HCI_EVT_LE_PER_ADV_SYNC_ESTABLISHED event is send to host (when first periodic advertising PDU is received) the ll_sync_create_cancel stopped to correctly. It can't be called after ull_sync_setup is called because scan->per_scan.sync is set to NULL in ull_sync_setup.

To Reproduce

1. Build direction_finding_connectionless_tx sample as an advertiser.
2. Build direction_finding_connectionless_rx sample as a scanned.
3. Run both.
4. Reset advertiser once sync is created reset advertiser multiple times with very short delays.
5. After periodic advertising sync timeouts there should be a message on scanner output that Deleting Periodic Advertising Sync... failed

Expected behavior
Periodic advertising should be successfully cancelled or deleted before BT_HCI_EVT_LE_PER_ADV_SYNC_ESTABLISHED event is generated.

Impact
Moderate, it is related with periodic advertising only.