platform/x86: fujitsu-laptop: distinguish current uses of device-specific data

In portions of the driver which use device-specific data, rename local
variables from fujitsu_bl and fujitsu_laptop to priv in order to clearly
distinguish these parts from code that uses module-wide data.

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