iio: dac: ad5686: Constify static struct iio_chan_spec

These are never modified and can be made const to allow the compiler to
put it in read-only memory.

Before:
   text    data     bss     dec     hex filename
   6642   12608      64   19314    4b72 drivers/iio/dac/ad5686.o

After:
   text    data     bss     dec     hex filename
  16946    2304      64   19314    4b72 drivers/iio/dac/ad5686.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>