iio: inkern: move to the cleanup.h magic

Use the new cleanup magic for handling mutexes in IIO. This allows us to
greatly simplify some code paths.

While at it, also use __free(kfree) where allocations are done and drop
obvious comment in iio_channel_read_min().

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240229-iio-use-cleanup-magic-v3-4-c3d34889ae3c@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>