greybus: es2: fix probe error handling

Make sure to return -ENODEV when the expected endpoints are missing and
stop relying on a default error.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>