iio: adc: xilinx: constify iio_buffer_setup_ops structure

The iio_buffer_setup_ops structures are never modified, so declare this one
as const, like the others.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>