iio: dac: ad5592r-base: Constify struct iio_chan_spec_ext_info

ad5592r_ext_info is not modified and can be made const to allow the
compiler to put it in read-only memory.

Before:
   text    data     bss     dec     hex filename
  13293    2088     256   15637    3d15 drivers/iio/dac/ad5592r-base.o

After:
   text    data     bss     dec     hex filename
  13421    1960     256   15637    3d15 drivers/iio/dac/ad5592r-base.o
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ad5592r-base.c

@@ -484,7 +484,7 @@ static ssize_t ad5592r_show_scale_available(struct iio_dev *iio_dev,
 		st->scale_avail[1][0], st->scale_avail[1][1]);
 }
 
-static struct iio_chan_spec_ext_info ad5592r_ext_info[] = {
+static const struct iio_chan_spec_ext_info ad5592r_ext_info[] = {
 	{
 	 .name = "scale_available",
 	 .read = ad5592r_show_scale_available,