ASoC: qdsp6: q6asm-dai: clean up a return

Smatch complains that if both "psubstream" and "csubstream" are NULL
then "ret" is uninitialized.  That probably can't happen, but it's
cleaner to just return zero anyway so let's do that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>