iio: addac: ad74xxx: Constify struct iio_chan_spec​

'struct iio_chan_spec' are not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  35749	   5879	    384	  42012	   a41c	drivers/iio/addac/ad74115.o
  32242	   3297	    384	  35923	   8c53	drivers/iio/addac/ad74413r.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  39109	   2519	    384	  42012	   a41c	drivers/iio/addac/ad74115.o
  33842	   1697	    384	  35923	   8c53	drivers/iio/addac/ad74413r.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/da291278e78b983ea2e657a25769f7d82ea2a6d0.1725717045.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>