Bluetooth: Mesh: Friend with unknown appkey

Ensures that friend messages are enqueued, even if the packet is
received with an appkey is unknown to the friend. Previously, sdu_recv
would return EINVAL if the appkey was unknown, which would prevent the
lower transport layer from adding the packet to the friend queue. This
is irrelevant for the logic in lower transport, and should not be
returned as an error.

Fixes #24014.

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