usb: renesas_usbhs: gadget: make usb_ep_ops const

Make the structure const as it is only stored in the ops field of a
usb_ep structure, which is of type const.
Done using Coccinelle.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>