dmaengine: sprd: Set cur_desc as NULL when free or terminate one dma channel

It will be failed to start one new transfer if the channel started one
none interrupt transfer before, since we will only set the schan->cur_desc
as NULL depending on the transfer interrupt now. Thus we should set
schan->cur_desc as NULL when free or terminate one dma channel to
avoid this issue.

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>