iio: consumers: Fix offset handling in iio_convert_raw_to_processed()

iio: consumers: Fix offset handling in iio_convert_raw_to_processed()

Fix iio_convert_raw_to_processed() offset handling for channels without
a scale attribute.

The offset has been applied to the raw64 value not to the original raw
value. Use the raw64 value so that the offset is taken into account.

Fixes: 14b457fdde38 ("iio: inkern: apply consumer scale when no channel scale is available")
Cc: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hansg@kernel.org>
Link: https://patch.msgid.link/20250831104825.15097-3-hansg@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>