usb: gadget: dnl: set dnl to be super speed

Set its max_speed to be super speed.

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