HID: roccat: add support for IskuFX

Extending isku module with one additional and one changed sysfs attr. IskuFX has
larger light sysfs attr. Made the code size tolerant so both devices can be
handled.

Signed-off-by: Stefan Achatz <erazor_de@users.sourceforge.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>