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

Use a managed input device for brightness 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>