usb: onboard-hub: Use the ofnode to check if the peer-hub was probed

usb: onboard-hub: Use the ofnode to check if the peer-hub was probed

Currently the check in usb_onboard_hub_bind is relying on specific
compatible string for the Michrochip USB5744. Replace this with
more generic approach that will allow to add new types of devices
to the of_match table. Because the driver only needs to bind one
"half" of the hub, the peer-hub node is used to find out if it
was already done. In case peer-hub was bound, -ENODEV is returned.

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Lukasz Czechowski <lukasz.czechowski@thaumatec.com>