iio: Replace 'sign' field with union in struct iio_scan_type

This field is used to differentiate between signed and unsigned integers.
A following commit will extend its use in order to add support for non-
integer scan elements; therefore, replace it with a union that contains a
more generic 'format' field. This union will be dropped when all drivers
are changed to use the format field.
Opportunistically replace character literals with symbolic constants that
represent the set of allowed values for the format field.

Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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