sh: Fix SH-4 DMAC CHCR masking.

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>