dmaengine: pl330: Improve transfer efficiency for the dregs

Only the unaligned burst transfers have the dregs.
so, still use BURST transfer with a reduced size
for better performance.

Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>
Link: https://lore.kernel.org/r/1593439555-68130-3-git-send-email-sugar.zhang@rock-chips.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>