dmaengine: pl330: Remove useless xfer_cb indirection

The xfer_cb callback of the pl330_req struct is always set to the same function.
This adds an unnecessary step of indirection. Instead just call the callback
function directly.

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