hp-wmi: check that an input device exists in resume handler

Some systems may not support input events, or registering the input
handler may have failed. So check that an input device exists before
trying to set the docking and tablet mode state during resume.

Fixes: http://bugzilla.kernel.org/show_bug.cgi?id=13865

Reported-and-tested-by: Cédric Godin <cedric@belbone.be>
Signed-off-by: Frans Pop <elendil@planet.nl>
Acked-by: Matthew Garrett <mjg59@srcf.ucam.org>
Signed-off-by: Len Brown <len.brown@intel.com>