dmaengine: sprd: Add validation of current descriptor in irq handler

When user terminates one DMA channel to free all its descriptors, but
at the same time one transaction interrupt was triggered possibly, now
we should not handle this interrupt by validating if the 'schan->cur_desc'
was set as NULL to avoid crashing the kernel.

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