fujitsu-laptop: use .notify method instead of installing hotkey handler directly

This patch adds a .notify() method.  The presence of .notify() causes
Linux/ACPI to manage event handlers and notify handlers on our behalf,
so we don't have to install and remove them ourselves.

Tested by Tony on Fujitsu-Siemens Lifebook S6420 [FJNB1E6] with
BIOS 1.18 (01/09/2009).  Tested by Jonathan on Fujitsu S7020.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Acked-By: Tony Vroon <tony@linx.net>
Tested-By: Tony Vroon <tony@linx.net>
Acked-by: Jonathan Woithe <jwoithe@physics.adelaide.edu.au>
Tested-by: Jonathan Woithe <jwoithe@physics.adelaide.edu.au>
Signed-off-by: Len Brown <len.brown@intel.com>