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: NFelipe Balbi <felipe.balbi@linux.intel.com>