iio: adc: constify iio_info structures

Declare iio_info structures as const as they are only stored in
the info field of a iio_dev structure. This field is of type const,
so iio_info structures having similar properties can be made const too.

File size before:
   text	   data	    bss	    dec	    hex	filename
   6944	    792	      0	   7736	   1e38	drivers/iio/adc/ti-ads1015.o

File size after:
   text   data	    bss	    dec	    hex	filename
   7264	    472	      0	   7736	   1e38	drivers/iio/adc/ti-ads1015.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>