staging: iio: ad5933: switch buffer mode to software

Since commit 152a6a88 ("staging:iio:accel:sca3000 move
to hybrid hard / soft buffer design.")
the buffer mechanism has changed and the
INDIO_BUFFER_HARDWARE flag has been unused.

Since commit 2d6ca60f ("iio: Add a DMAengine framework
based buffer")
the INDIO_BUFFER_HARDWARE flag has been re-purposed for
DMA buffers.

This driver has lagged behind these changes, and
in order for buffers to work, the INDIO_BUFFER_SOFTWARE
needs to be used.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Fixes: 2d6ca60f ("iio: Add a DMAengine framework based buffer")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>