Commit 2436bdcd ("dma-coherent: remove the DMA_MEMORY_MAP and
DMA_MEMORY_IO flags") missed to remove the 'dma' variable causing
the following build warning:

arch/arm/mach-imx/mach-mx31moboard.c:478:6: warning: unused variable 'dma' [-Wunused-variable]

Remove the unused 'dma' variable.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>