Bluetooth: add missing initilization of sync_info member

During manual testing with UBSAN enabled, it was found that
bt_le_per_adv_sync_cb synced() callback in application
sometimes gets unexpected different values in
`sync_info.recv_enabled` field.

The UBSAN output contained the following warnings message:
runtime error:
load of value 65, which is not a valid value for type '_Bool'

It tunrned out that struct bt_le_per_adv_sync_synced_info sync_info
inside bt_hci_le_past_received_common() contains uninitialized
recv_enabled member.
Set it to true by default to avoid possible issues.

Signed-off-by: Ivan Iushkov <ivan.iushkov@nordicsemi.no>