iio: buffer: correctly return bytes written in output buffers

iio: buffer: correctly return bytes written in output buffers

If for some reason 'rb->access->write()' does not write the full
requested data and the O_NONBLOCK is set, we would return 'n' to
userspace which is not really truth. Hence, let's return the number of
bytes we effectively wrote.

Fixes: 9eeee3b0bf190 ("iio: Add output buffer support")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20230216101452.591805-2-nuno.sa@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>