When formatting a value using IIO_VAL_FRACTIONAL_LOG2 and the values is
between -1 and 0 the sign is omitted.

We need the same trick as for IIO_VAL_FRACTIONAL to make sure this gets
formatted correctly.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20201215191743.2725-2-lars@metafoo.deSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>