platform/x86: fujitsu-laptop: use a sparse keymap for hotkey event generation

Simplify hotkey event generation by using a sparse keymap.  As sparse
keymap operates on scancodes instead of keycodes, adjust variable names
and debug messages accordingly.

This patch only handles the default keymap for clarity.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Reviewed-by: Jonathan Woithe <jwoithe@just42.net>
[dvhart: correct flag passed to call_fext_func in acpi_fujitsu_laptop_notify]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>