caif: Bugfix - XOFF removed channel from caif-mux

XOFF was mixed up with DOWN indication, causing causing CAIF channel to be
removed from mux and all incoming traffic to be lost after receiving flow-off.
Fix this by replacing FLOW_OFF with DOWN notification.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>