ASoC: qdsp6: q6routing: add proper error check

q6adm_open can return error pointer or a null in error cases.
Fix the return handling.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Acked-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>