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>