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.

Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Link: https://lore.kernel.org/r/20220708170153.269991-4-ben.dooks@sifive.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>