dmaengine: omap-dma: consolidate clearing channel status register

Consolidate clearing of the channel status register, rather than open
coding the same functionality in two places.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>