ucm: fix the logic of choosing the default cdev

ucm: fix the logic of choosing the default cdev

If the cdev has not been configured explicitly, use the PlaybackCTL
or CaptureCTL value if one of them is set. If neither are set, or if
both are set to different values, then there's no sensible default, so
executing the sequence should fail. The previous code probably tried
to implement this logic, but it was buggy.

Also use more descriptive variable names than "cdev1" and "cdev2".

Signed-off-by: Tanu Kaskinen <tanu.kaskinen@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>