iio: consumers: Add an iio_multiply_value() helper function

The channel-scale handling in iio_convert_raw_to_processed() in essence
does the following:

processed  = raw * caller-provided-scale * channel-scale

Which can also be written as:

multiplier = raw * caller-provided-scale
iio-value  = channel-scale
processed  = multiplier * iio-value

Where iio-value is a set of IIO_VAL_* type + val + val2 integers, being
able to handle multiplication of iio-values like this is something
which is useful to have in general and, as previous bugfixes to
iio_convert_raw_to_processed() have shown, also tricky to implement.

Split the iio-value multiplication code from iio_convert_raw_to_processed()
out into a new iio_multiply_value() helper. This serves multiple purposes:

1. Having this split out allows writing a KUnit test for this.
2. Having this split out allows re-use to get better precision
   when scaling values in iio_read_channel_processed_scale().

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hansg@kernel.org>
Link: https://patch.msgid.link/20250831104825.15097-4-hansg@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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