iio: backend: remove unused parameter

Indio_dev was not being used in iio_backend_extend_chan_spec() so remove
it.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240709-dev-iio-backend-add-debugfs-v1-1-fb4b8f2373c7@analog.com


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