ASoC: core - pcm mutex per rtd

In preparation for ASoC DSP support.

The new DSP core allows DSP DAIs to be dynamically re-routed at runtime
between the PCM device end (or Frontend - FE) and the physical DAI
(Backend - BE) using regular kcontrols (just like a hardware CODEC).
The DSP core therefore must be able to call PCM operations for both the
Frontend and Backend(s) DAIs at the same time.

Currently we have a global pcm_mutex that is used to serialise
the ASoC PCM operations. This patch removes the global mutex
and adds a mutex per RTD allowing the PCM operations to be reentrant and
allow control of more than one DAI at at time. e.g. a frontend DSP hw_params()
could configure multiple backend hw_params() with similar or different
hw parameters at the same time.

Also fix the naming of soc_pcm_close.

Signed-off-by: Liam Girdwood <lrg@ti.com>