dmaengine: mv_xor_v2: convert callback to helper function

This is in preparation of moving to a callback that provides results to the
callback for the transaction. The conversion will maintain current behavior
and the driver must convert to new callback mechanism at a later time in
order to receive results.

Signed-off-by: Hanna Hawa <hannah@marvell.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>