kset: convert drivers/base/class.c to use kset_create

Dynamically create the kset instead of declaring it statically.

The class_obj subsystem is not yet converted as it is more complex and
should be going away soon with the removal of class_device from the
kernel tree.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>