dmaengine: pl330: Use dma_transfer_direction instead of custom pl330_reqtype

The pl330 driver has the custom pl330_reqtype enum which has the same possible
settings as the generic dma_transfer_direction enum. Switching over to the
generic enum internally makes it possible to directly initialize it from the
transfer request direction.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>