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>