usb: dwc3: gadget: return 0 if we try to Wakeup in superspeed

Instead of returning -EINVAL when someone calls
__dwc3_gadget_wakeup() in speeds > highspeed, let's
return 0. There are no problems for the driver for
calling it in superspeed as we cleanly just return.

This avoids an annoying WARN_ONCE() always
triggering during superspeed enumeration with LPM
enabled.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>