driver core: add __printf verification to device_create_groups_vargs

__printf is useful to verify format and arguments. Remove the following
warning (with W=1):

  drivers/base/core.c:2435:2: warning: function might be possible candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
Signed-off-by: NMathieu Malaterre <malat@debian.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/core.c

@@ -2411,7 +2411,7 @@ static void device_create_release(struct device *dev)
 	kfree(dev);
 }
 
-static struct device *
+static __printf(6, 0) struct device *
 device_create_groups_vargs(struct class *class, struct device *parent,
 			   dev_t devt, void *drvdata,
 			   const struct attribute_group **groups,