iio: core: Match iio_device_claim_*() semantics and implementation

iio: core: Match iio_device_claim_*() semantics and implementation

Implement iio_device_claim_buffer_mode() fully inline with the use of
__iio_dev_mode_lock(), which takes care of sparse annotations.

To completely match iio_device_claim_direct() semantics, we need to
also change iio_device_claim_buffer_mode() return semantics to usual
true/false conditional lock semantics.

Additionally, to avoid silently breaking out-of-tree drivers, rename
iio_device_claim_buffer_mode() to iio_device_claim_try_buffer_mode().

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Kurt Borja <kuurtb@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>