greybus: connection: fix lookup race

Fix longstanding race that could lead to a connection being destroyed
while processing an incoming message due to missing reference counting
when looking up the connection.

Add helpers to manipulate the connection kref.

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>