usb: host: dwc3-sti-glue: Use UCLASS_NOP instead of UCLASS_MISC

dwc3-sti-glue 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: Patrice Chotard <patrice.chotard@st.com>