dmaengine: dw-axi-dmac: do not print NULL LLI during error

During debugging we have seen an issue where axi_chan_dump_lli()
is passed a NULL LLI pointer which ends up causing an OOPS due
to trying to get fields from it. Simply print NULL LLI and exit
to avoid this.

Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Link: https://lore.kernel.org/r/20220708170153.269991-3-ben.dooks@sifive.com


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