kobject: convert efivars to kobj_attr interface

This cleans up a lot of code and gets rid of a unneeded macro, and gets
us one step closer to deleting the deprecated subsys_attr code.

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>