Bluetooth: Mesh: Friend SeqAuth cleanup

The Friend queue uses the message SeqAuth to determine whether the
message is already in the queue. To facilitate this, the SeqAuth is
passed around as a pointer throughout the transport modules. In the
bt_mesh_ctl_send functions, this parameter is also exposed in the API,
but the internal usage is inconsistent and buggy. Also, no one actually
uses this parameter.

- Removes seq_auth param from bt_mesh_ctl_send, instead passing NULL
  directly to the friend module, to enforce its addition to the queue.
- Makes the seq_auth pointer const throughout the friend module.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>