ASoC: Add control_type in snd_soc_codec

This is mainly used by the soc-cache code to easily determine the
currently used underlying serial bus.  Set SND_SOC_CUSTOM to 1 so we
can distinguish it if it is not initialized or set.

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