mwan3: Fix json_load fails with some data

Sometimes the return value of `ubus -S call network.interface.wan status`
cause `json_load` to return `Failed to parse message data` error.

To avoid this, the JSON data always should be quoted with double quotes.

Signed-off-by: Evren Yurtesen <eyurtese@abo.fi>
Removed quoatation marks from commit heading
Signed-off-by: Florian Eckert <fe@dev.tdt.de>