greybus: audio: Avoid reporting spurious button events

Now jack-button are registered to snd framework based on capabilities
populated by codec module's topology data. Thus, valid ids for button
events can also vary for different modules. This patch modifies existing
button reporting mechanism to avoid reporting spurious button events for
invalid button ids.

Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>