HID: hid-lg4ff: Store pointer to the output HID report struct in the device entry struct

Store pointer to the output HID report struct in the device entry struct.
This eliminates the need to look the HID report struct up every time it is needed.
Signed-off-by: Michal Malý <madcatxster@devoid-pointer.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>