iio: dac: ad5380: Constify struct iio_chan_spec_ext_info

ad5380_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
  12060    3280     192   15532    3cac drivers/iio/dac/ad5380.o

After:
   text    data     bss     dec     hex filename
  12252    3088     192   15532    3cac drivers/iio/dac/ad5380.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/ad5380.c

@@ -240,7 +240,7 @@ static const struct iio_info ad5380_info = {
 	.write_raw = ad5380_write_raw,
 };
 
-static struct iio_chan_spec_ext_info ad5380_ext_info[] = {
+static const struct iio_chan_spec_ext_info ad5380_ext_info[] = {
 	{
 		.name = "powerdown",
 		.read = ad5380_read_dac_powerdown,