staging: comedi: dt282x: remove sanity checks of 's->async->prealloc_buf'

The comedi core sanity checks that the subdevice has a buffer allocated
before allowing an async command to start. The helper functions in comedi_buf
will also sanity check the buffer as a side effect of the 'alloc' helpers
with reading or writing data for the buffer.

Remove the unnecessary sanity checks in the ai/ao dma interrupt handlers.
This will allow the handlers to disable the dma and then fail when they
try to read/write the buffer. Currently dma is left enabled due to the
sanity checks.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>