ASoC: Clean up docbook warnings

A number of functions and structures in the sound subsystem had incomplete
and/or obsolete DocBook comments, leading to warnings when the docs were
built.  Correct those comments so that we can enjoy our audio in the
absence of warning noise.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mark Brown <broonie@kernel.org>