staging: comedi: adl_pci9118: change type of pci9118_dmabuf 'virt' member

For aesthetics, change the type of this member to avoid the casts when allocating
and freeing the DMA buffers. This does introduce a cast in move_block_from_dma()
but that cast is cleaner.

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>