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: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>