usb: gadget: aspeed-vhub: constify usb_gadget_ops structure

The usb_gadget_ops structure can be const as it is only stored in
the ops field of a usb_gadget structure and this field is const.

Done with the help of Coccinelle.

Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>