ASoC: soc-dai: return proper error for get_sdw_stream()

snd_soc_dai_get_sdw_stream() returns null if dai does not support
this callback, this is no very useful for the caller to
differentiate if this is an error or unsupported call for the dai.

return -ENOTSUPP in cases where this callback is not supported.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20200316151110.2580-1-srinivas.kandagatla@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>