iio:adc:berlin2-adc: constify iio_chan_spec

Mark berlin2_adc_channels array as constant.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>