mmc: omap_hsmmc: remove private DMA API implementation

Remove the private DMA API implementation from omap_hsmmc, making it
use entirely the DMA engine API.
Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>