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: 3120910a

 ("ARM: dts: stih407-family: Harmonize DWC USB3 DT nodes name")
Reported-by: Jerome Audu <jerome.audu@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Link: https://lore.kernel.org/r/20220926124359.304770-1-patrice.chotard@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>