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>