dmaengine: dw-axi-dmac: ignore interrupt if no descriptor

In the Linux kernel, the following vulnerability has been resolved:

dmaengine: dw-axi-dmac: ignore interrupt if no descriptor

If the channel has no descriptor and the interrupt is raised then the

kernel will OOPS. Check the result of vchan_next_desc() in the handler

axi_chan_block_xfer_complete() to avoid the error happening.