usb: dwc3: st: Fix node's child name

Update node's child name from "dwc3" to "usb", this fixes
the following issue:

[3.773852] usb-st-dwc3 8f94000.dwc3: failed to find dwc3 core node

Fixes: 3120910a099b ("ARM: dts: stih407-family: Harmonize DWC USB3 DT nodes name")
Reported-by: Jerome Audu <[e-mail]>
Signed-off-by: Patrice Chotard <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/dwc3/dwc3-st.c

···
 	/* Manage SoftReset */
 	reset_control_deassert(dwc3_data->rstc_rst);
 
-	child = of_get_child_by_name(node, "dwc3");
+	child = of_get_child_by_name(node, "usb");
 	if (!child) {
 		dev_err(&pdev->dev, "failed to find dwc3 core node\n");
 		ret = -ENODEV;