dmaengine: sprd: Fix the possible crash when getting descriptor status

We will get a NULL virtual descriptor by vchan_find_desc() when the descriptor
has been submitted, that will crash the kernel when getting the descriptor
status.

In this case, since the descriptor has been submitted to process, but it
is not completed now, which means the descriptor is listed into the
'vc->desc_submitted' list now. So we can not get current processing descriptor
by vchan_find_desc(), but the pointer 'schan->cur_desc' will point to the
current processing descriptor, then we can use 'schan->cur_desc' to get
current processing descriptor's status to avoid this issue.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>