ASoC: core: Log a warning when machines use soc-audio

snd_soc_register_card() has been available and strongly preferred since
2.6.38 but we're still seeing new drivers using it and the conversion rate
for older machines has been low. Help address both issues by logging a
warning when the soc-audio device probes.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>