ARM: S3C24XX: dma-s3c2443 - do not write into arbitary bits

The values read from the channel map always also contain the
DMA_CH_VALID (= 1<<31) setting, which should not get written
into the register, even if this bit is unused.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>