iio: buffer-dmaengine: Use dmaengine_terminate_sync()

The DMAengine framework gained support for synchronized transfer
termination. Use the new dmaengine_terminate_sync() function instead of
dmaengine_terminate_all(), this avoids a potential race condition when
disabling the buffer.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>