ASoC: SOF: sof-client: Enforce client access scope

Merge series from Peter Ujfalusi <peter.ujfalusi@linux.intel.com>:

Up to this point we had a spoken rule what sof-client can access and what
they must not (they cannot use sdev directly), recently there were attempts
to break this rule as clients could access sdev via the sof client dev.

The series will re-arrange the client device struct to 'hide' the sdev (and
the list) from client drivers to prevent abuse.