platform/x86: fujitsu-laptop: Simplify soft key handling

The hardcoded BIT(X) used in the soft key handling can be confusing and
prone to errors.

Instead, use the status FLAG_* defines for the sparse keymap index.
Rather than check for each known bit, use a bitmask to filter for all
known soft keys, and use the for_each_set_bit iterator.

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