iio: afe: rescale: add offset support

[ Upstream commit a29c3283

 ]

This is a preparatory change required for the addition of temperature
sensing front ends.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220213025739.2561834-4-liambeguin@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Stable-dep-of: bee44839

 ("iio: afe: rescale: Accept only offset channels")
Signed-off-by: Sasha Levin <sashal@kernel.org>