greybus: connection: convert connected dev_warn() to dev_err()

Failures from control-connected operations are fatal errors and
must be reported with dev_err() instead of dev_warn().

Fix it.

Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
[ johan: do not promote disconnected warnings, update summary ]
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>