usb: gadget: configfs: make config_item_type const

Make config_item_type structures const as they are either passed to a
function having the argument as const or stored in the const "ci_type"
field of a config_item structure.

Done using Coccinelle.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>