usb: dwc3: Use UCLASS_NOP instead of UCLASS_MISC for the DWC3 generic glue

dwc3-generic has been broken since MISC uclass has been modified to scan DT
sub-nodes after bind.
Fixing it by a using the no-op uclass.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>