staging:iio: Constify static iio_chan_spec arrays

The per driver iio_chan_spec arrays are usually shared between multiple device
instances. So a single device instance may not modify the iio_chan_spec array
since this would also affect the other device instances. To make this restriction
explicit mark the per driver iio_chan_spec arrays as const.

Conversion was done automatically using the following coccinelle semantic patch:

// <smpl>
@disable optional_qualifier@
identifier channels;
@@
static
+const
struct iio_chan_spec channels[] = ...;
// </smpl>

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