iio: iio_format_value(): Use signed temporary for IIO_VAL_FRACTIONAL_LOG2

IIO_VAL_FRACTIONAL_LOG2 works with signed values, yet the temporary we use
is unsigned. This works at the moment because the variable is implicitly
cast to signed everywhere where it is used.

But it will certainly be cleaner to use a signed variable in the first
place.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20201215191743.2725-1-lars@metafoo.de


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>