usb: host: dwc3-sti-glue: Fix ofnode_valid() parameter

node varaible is used as iterator into ofnode_for_each_subnode()
loop, when exiting of it, node is no more a valid ofnode.
Use dwc3_node instead as parameter of ofnode_valid()

Fixes: ac28e59a ("usb: Migrate to support live DT for some driver")
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Cc: Kever Yang <kever.yang@rock-chips.com>