usb: cdns3: gadget: Configure speed in udc_start

When one of the functions does not support super speed, the composite
driver forces the gadget to high speed. But the speed is never
configured in the cdns3 gadget driver. So configure the speed
in cdns3_gadget_udc_start just like in the kernel.

Signed-off-by: Ravi Gunasekaran <r-gunasekaran@ti.com>
Reviewed-by: Marek Vasut <marex@denx.de>