greybus: manifest: don't need to check for desc_size == 0 anymore

identify_descriptor() doesn't return 0 anymore and so we don't need to check the
returned value against 0.

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