staging: comedi: addi_apci_3120: switch DMA buffers after writing samples

Currently the DMA buffers are switched before writing the current samples to
the async buffer. This works but when the EOA event happens we end up with an
outstanding DMA operation in progress that gets terminated by the (*cancel).

Avoid the outstanding DMA operation by switching the DMA buffers after writing
the samples. The driver will detect the EOA event and not retart the DMA.

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>