iio: Replace occurrences of magic number 0 by IIO_CHAN_INFO_RAW

Usually, functions responsible for reading raw data typically relies on
values from iio_chan_info_enum to correctly identify the type of data to
be read. There is a set of a device driver that uses the magic number 0
instead of IIO_CHAN_INFO_RAW. This patch improves the readability by
replaces the magic number 0 for the appropriate IIO_CHAN_INFO_RAW in six
devices driver in the IIO subsystem.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>