platform/x86: fujitsu-laptop: clearly denote backlight-related symbols

Unify naming for all backlight-related functions, structures, variables
and constants by using a consistent "_bl"/"_BL" suffix/infix.  Adjust
indentation to make checkpatch happy.
Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
[kempniu: rebase patch, rewrite commit message]
Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Jonathan Woithe <jwoithe@just42.net>