usb: dwc3: ulpi: make dwc3_ulpi_ops constant

ulpi_register_interface() accepts a const struct ulpi_ops and dwc3
doesn't perform any changes to this struct at runtime, so there's no
reason it shouldn't be constant.
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Tal Shorer <tal.shorer@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/ulpi.c

@@ -65,7 +65,7 @@ static int dwc3_ulpi_write(struct device *dev, u8 addr, u8 val)
 	return dwc3_ulpi_busyloop(dwc);
 }
 
-static struct ulpi_ops dwc3_ulpi_ops = {
+static const struct ulpi_ops dwc3_ulpi_ops = {
 	.read = dwc3_ulpi_read,
 	.write = dwc3_ulpi_write,
 };