staging: comedi: adl_pci9118: simplify interrupt_pci9118_ai_dma() a bit

Eliminate the `next_dma_buf` variable in `interrupt_pci9118_ai_dma()`.
It holds the next value of `devpriv->dma_actbuf` when double buffering
is used, but we can just set that to the next value directly at the
point where the buffers are switched as the old value is not used
anywhere else.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>