iio: adc: max11100: Constify struct iio_chan_spec

max11100_channels is not modified and can therefore be made const to
allow the compiler to put it in read-only memory.

Before:
   text    data     bss     dec     hex filename
   3776    1168       0    4944    1350 drivers/iio/adc/max11100.o

After:
   text    data     bss     dec     hex filename
   3968     976       0    4944    1350 drivers/iio/adc/max11100.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>