greybus: audio: Add check for invalid index while mapping control

While mapping control id to define DAPM routes, invalid control index
may cause kernel oops. Add extra check to validate index while mapping
names to control_id.

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>