platform/x86: fujitsu-laptop: simplify brightness key event generation logic

Returning early when there is no brightness change allows removal of a
duplicate code block, makes the purpose of the following code clearer
and allows the condition surrounding key event generation to be removed.
Local integer variables can also be declared in a single line.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-and-tested-by: Jonathan Woithe <jwoithe@just42.net>