platform/x86: fujitsu-laptop: refactor backlight input device setup

Simplify error handling in acpi_fujitsu_bl_add() by moving code
responsible for setting up the input device to a separate function.

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