usb: gadget: configfs: Constify struct config_item_type

usb: gadget: configfs: Constify struct config_item_type

'struct config_item_type' is not modified in this file.

Apparently, these structures are only used with
config_group_init_type_name() which takes a const struct config_item_type*
as a 3rd argument.

Constifying this structure moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  40834	   5112	     64	  46010	   b3ba	drivers/usb/gadget/configfs.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  41218	   4728	     64	  46010	   b3ba	drivers/usb/gadget/configfs.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/513223e97082e1bb758e36d55c175ec9ea34a71c.1723323896.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>