iio: inkern: fix iio_convert_raw_to_processed_unlocked

When reading IIO_CHAN_INFO_OFFSET, the return value of iio_channel_read() for
success will be IIO_VAL*, checking for 0 is not correct.

Without this fix the offset applied by iio drivers will be ignored when
converting a raw value to one in appropriate base units (e.g mV) in
a IIO client drivers that use iio_convert_raw_to_processed including
iio-hwmon.

Cc: <stable@vger.kernel.org> # 3.10.x
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>