net: qrtr: Broadcast DEL_CLIENT message when endpoint is closed

Per the QMUXv2 protocol specificiation a DEL_CLIENT message should be
broadcasted when an endpoint is disconnected.

The protocol specification does suggest that the router can keep track
of which nodes the endpoint has been communicating with to not wake up
sleeping remotes unecessarily, but implementation of this suggestion is
left for the future.

Cc: Courtney Cavin <ccavin@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>