platform/x86: fujitsu-laptop: switch to a managed hotkey input device

Use a managed input device for hotkey events in order to simplify two
error paths and one cleanup function while also reducing the number of
local variables required.  Remove double assignment to make checkpatch
happy.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Reviewed-by: Jonathan Woithe <jwoithe@just42.net>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>