dmaengine: sprd: Fix the last link-list configuration

We will pass sglen as 0 configure the last link-list configuration
when filling the descriptor, which will cause the incorrect link-list
configuration. Thus we should check if the sglen is 0 to configure
the correct link-list configuration.

Signed-off-by: Eric Long <eric.long@spreadtrum.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>