usb: dwc2: gadget: Program ep0_mps for LS

When device is enumerated in LS we should program ep0_mps accordingly.
USB2 spec says that in LS mode, control ep mps must be 8.

Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>