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>

drivers/staging/greybus/connection.c

@@ -386,7 +386,7 @@ static int gb_connection_init(struct gb_connection *connection)
 
 		ret = gb_control_connected_operation(control, cport_id);
 		if (ret) {
-			dev_warn(&connection->dev,
+			dev_err(&connection->dev,
 				"Failed to connect CPort-%d (%d)\n",
 				cport_id, ret);
 			return 0;