greybus: manifest: fix illegal free in error path

The manifest-parsing code could end up leaving the interface
vendor_string set to an error pointer that we'd eventually try to free
when destroying the interface.

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>