osmux: Lower log level when osmux batch received for unknown CID

This is actually quite common, since our peer may be sending some osmux
packets to us a while after we have closed the conn on our side,
specially if latency is high in the network (eg satellite links).

Related: SYS#5987
Change-Id: Idffd91b68d1f98ac906d78e7fbc0e6eaa1962f9e
1 file changed