iio: buffer-dma: Fix docstrings

Fix a typo in the docstring of iio_dma_buffer_read() and fix what looks
like a copy-and-paste error in the iio_dma_buffer_write() docstring.

Signed-off-by: Folker Schwesinger <dev@folker-schwesinger.de>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/D83IPSTKYWNB.1PUBV1530XI86@folker-schwesinger.de


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