firmware: vpd: do not leave freed section attributes to the list

We should only add section attribute to the list of section attributes
if we successfully created corresponding sysfs attribute.

Fixes: 049a59db

 ("firmware: Google VPD sysfs driver")
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>