iio: Adjust internals of handling of direct mode claiming to suit new API.

Now there are no remaining callers of iio_device_claim_direct_mode()
and iio_device_release_direct_mode() rename those functions to ensure
they are not used in new drivers. Also make them now return booleans
in line with the sparse friendly static inline wrappers.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://patch.msgid.link/20250331121317.1694135-38-jic23@kernel.org


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