iio: dac: ad5380: Constify struct iio_chan_spec_ext_info

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>