iio: buffer-dmaengine: adjust `bytes_used` with residue info

iio: buffer-dmaengine: adjust `bytes_used` with residue info

A transfer may fall shorter than the bytes in the block.
This information is available in the residue from the DMA engine, so we can
compute actual `bytes_used` with that by subtracting the residue.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20200826052011.13348-1-lars@metafoo.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>