iio: buffer-dmaengine: Enable write support

Use the iio_dma_buffer_write() and iio_dma_buffer_space_available()
functions provided by the buffer-dma core, to enable write support in
the buffer-dmaengine code.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240419-iio-backend-axi-dac-v4-5-5ca45b4de294@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>