iio: core: Consolidate iio_format_avail_{list,range}()

The iio_format_avail_list() and iio_format_avail_range() functions are
almost identical. The only differences are that iio_format_avail_range()
expects a fixed amount of items and adds brackets "[ ]" around the output.

Refactor them into a common helper function. This improves the
maintainability of the code as it makes it easier to modify the
implementation of these functions.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20201114120000.6533-1-lars@metafoo.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>