kset: convert efivars to use kset_create for the vars sub-subsystem.

Dynamically create the kset instead of declaring it statically.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Matt Domsch <Matt_Domsch@dell.com>
Cc: Matt Tolentino <matthew.e.tolentino@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>