Bluetooth: Mesh: Demo: Increase loopback buf count

Increases the loopback buffer count to 8 in the mesh demo to give it
enough room for buffering the configuration messages in the
initialization step. As these messages are sent in a non-blocking
manner, they'll all be queued up for execution before any of them is
processed.

Fixes #30207.

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