iio:adc:ti_am335x_adc Fix buffered mode by identifying as software buffer.

Whilst this part has a hardware buffer, the identifcation that IIO cares
about is the userspace facing end.  It this case we push individual elements
from the hardware fifo into the software interface (specifically a kfifo)
rather than providing direct reads through to a hardware buffer
(as we still do in the sca3000 for example).

Technically the original specification as a hardware buffer could be
considered wrong, but it didn't matter until the patch listed below.

Result is that any attempt to enable the buffer will return -EINVAL

Fixes: 225d59ad ("iio: Specify supported modes for buffers")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>