iio: core: Constify struct configfs_item_operations and configfs_group_operations

iio: core: Constify struct configfs_item_operations and configfs_group_operations

'struct configfs_item_operations' and 'configfs_group_operations' are not
modified in this driver.

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

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   5037	   1528	     64	   6629	   19e5	drivers/iio/industrialio-sw-device.o
   5509	   1528	     64	   7101	   1bbd	drivers/iio/industrialio-sw-trigger.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   5133	   1432	     64	   6629	   19e5	drivers/iio/industrialio-sw-device.o
   5605	   1432	     64	   7101	   1bbd	drivers/iio/industrialio-sw-trigger.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>