ARM: S3C64XX: Clear DMA flags on channel request

This patch clears the DMA flags when a DMA channel is requested. This is
necessary because otherwise the channel may inherit incompatible
settings from its last usage.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>