usb: mtu3: add ->udc_set_speed()

Make use of the method to make sure connect on speeds
supported by the gadget driver.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1595834101-13094-6-git-send-email-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>