iio: inkern: fix channel read regression

A recent "cleanup" broke IIO channel read outs and thereby thermal
mitigation on the Lenovo ThinkPad X13s by returning zero instead of the
expected IIO value type in iio_read_channel_processed_scale():

	thermal thermal_zone12: failed to read out thermal zone (-22)

Fixes: 3092bde7

 ("iio: inkern: move to the cleanup.h magic")
Cc: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20240530074416.13697-1-johan+linaro@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>