firmware: export firmware_kset so that people can use that instead of the braindead firmware_register interface

Needed for future firmware subsystem cleanups.

In the end, the firmware_register/unregister functions will be deleted
entirely, but we need this symbol so that subsystems can migrate over.

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>