prepare callback can be called multiple times, so unprepare the stream
if its already prepared.

Without this DSP is not happy to setting the params on a already
prepared graph.

Fixes: 9b4fe0f1 ("ASoC: qdsp6: audioreach: add q6apm-dai support")
Reported-by: Srinivasa Rao Mandadapu <quic_srivasam@quicinc.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220610144818.511797-1-srinivas.kandagatla@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>