platform/chrome: typec: Fix ret value check error

cros_typec_add_partner() returns 0 on success, so check for "ret"
instead of "!ret" as an error.

Signed-off-by: Prashant Malani <pmalani@chromium.org>
Fixes: 9d33ea33 ("platform/chrome: cros_ec_typec: Register port partner")
Signed-off-by: Benson Leung <bleung@chromium.org>