iio: core: Fix mapping of iio channels to entry numbers

When adding maps to the list, they were added using list_add, which adds
them in LIFO order. When parsing using iio_channel_get_all(), these
elements are hence returned in reverse order. As a result, the iio_hwmon
mapping maps the first entry to the last channel and so on.

Signed-off-by: Gaurav Gupta <gauragup@cisco.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>