iio: iio: Fix iio_channel_read return if channel havn't info

iio: iio: Fix iio_channel_read return if channel havn't info

[ Upstream commit 65de7654d39c70c2b942f801cea01590cf7e3458 ]

When xilinx-xadc is used with hwmon driver to read voltage, offset used
for temperature is always applied whatever the channel.

iio_channel_read must return an error to avoid offset for channel
without IIO_CHAN_INFO_OFFSET property.

Signed-off-by: Fabien Proriol <fabien.proriol@jdsu.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>