topology: pre-process-object: expand variables before validating attributes

topology: pre-process-object: expand variables before validating attributes

With the introduction of variables in the topology files, validation of
attributes values must be done after they are expanded to their defined
values. Also, since valid values for attributes in the class definition
can also be variables, they need to be expanded as well. So, first expand
the attribute values and then check them against expanded valid values.

Fixes: https://github.com/alsa-project/alsa-utils/pull/138
Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>