iio: inkern: Convert iio_read_avail_channel_raw into a wrapper

Convert "iio_read_avail_channel_raw" over to a wrapper around
"iio_read_avail_channel_attribute".

With the introduction of "iio_read_avail_channel_attribute",
the necessity of having a separate call to read raw channel values
became redundant.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>