iio: use IIO_CHAN_INFO_RAW rather than 0

(a) For better readability, replace 0 with IIO_CHAN_INFO_RAW.
(b) Make same line-format as other apis()
    : iio_read_channel_scale() and iio_read_channel_offset()

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>