usb: gadget: set correct usb_configuration for os_desc_config

The current way to set cdev->os_desc_config is wrong if user restart
fastboot, as the old config is not used anymore and new allocated
usb_configuration will be used, so set the os_desc_config while
usb_add_config.

Reviewed-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>