iio: inkern: split of_iio_channel_get_by_name()

This change splits of_iio_channel_get_by_name() so that it decouples
looking for channels in the current node from looking in it's parents
nodes. This will be helpful when moving to fwnode properties where we
need to release the handles when looking for channels in parent's nodes.

No functional change intended...

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220715122903.332535-5-nuno.sa@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>