This patch fixes the DMA cascade by masking the correct bits.

Tested and working with Dreamcast PVR2 DMA. With this patch applied
the existing mainline code in arch/sh/drivers/dma/dma-sh.c works,
whereas before I was patching that to get round this problem.
Signed-off-by: Adrian McMenamin <adrian@mcmen.demon.co.uk>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>