audioctl: don't return failure because of illegal enum value
audioctl(1) currently has a problem if the audio driver gives it an invalid enum value in one of the controls. This can happen because of #10617. The situation right now is that audioctl(1) will stop in the middle of a show-controls or save-controls command with an error message that isn't all that helpful to a user, and in case of save-controls it'll save the controls only up to this point.
I consider the bad enum value only a minor issue, nothing that should prevent audioctl(1) to stop working properly. Just ignoring the invalid control seems like a saner choice than to error out in the middle of processing controls.