iio: buffer-dma: Rename iio_dma_buffer_data_available()

Change its name to iio_dma_buffer_usage(), as this function can be used
both for the .data_available and the .space_available callbacks.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240419-iio-backend-axi-dac-v4-2-5ca45b4de294@analog.com


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