ASoC: when removing a CPU DAI, clean up its DAPM context

When a standalone CPU DAI (one not part of a CODEC) is probed, widgets
are created for it. Add a call to snd_soc_dapm_free() in order to clean
these up when the CPU DAI is removed.

In order for snd_soc_dapm_free() to work, the CPU DAI's DAPM context's
list member must be initialized, since snd_soc_dapm_free() removes that
from the list it's part of. Add it to the card's list of DAPM contexts.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>