platform/x86: fujitsu-laptop: make disable_brightness_adjust a boolean

Due to the way the disable_brightness_adjust module parameter is
currently handled in acpi_fujitsu_bl_add(), it can only be set to either
0 or 1, which effectively makes it a boolean.  Emphasize that by
changing module parameter type to bool.  Do not announce parameter value
in a debug message as it can be dynamically changed via sysfs and its
current value can also be read from there.  Clean up module parameter
description.

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