staging:iio:ad7887: Use passed in chan spec in ad7887_read_raw

Use the passed in chan spec in ad7887_read_raw instead of alawys using the first
chan spec entry from the chip info data. Since all channels have the same shift
and realbits from a functional point of view it does not matter which chan spec
is used, but the patch makes the a bit more clear.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>