staging: comedi: das1800: separate AI FIFO bounce buffer from DMA buffer

This driver can use DMA or the hardware FIFO to read analog input samples.
When DMA is enabled it's also possible for some async commands to cause
DMA to be disabled and the FIFO is used instead.

Currently the first DMA channels buffer is used for the bounce buffer
when using the FIFO. For aesthetics, add a new member to the private
data and use that for the FIFO bounce buffer.

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>