The dummy driver uses runtime->private_free but still frees
its pcm structures on error paths.

This is esoteric because the error paths in question are
unreachable. Thus the bug is only a problem when someone
copies this code into other drivers.
Signed-off-by: NDaniel R Thompson <daniel.thompson@st.com>
Signed-off-by: NJaroslav Kysela <perex@perex.cz>