iio: core: Simplify iio_format_list()

iio_format_list() has two branches in a switch statement that are almost
identical. They only differ in the stride that is used to iterate through
the item list.

Consolidate this into a common code path to simplify the code.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20201114120000.6533-2-lars@metafoo.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>