HID: logitech-dj: add support for the static device in the Powerplay mat/receiver

The Logitech G Powerplay has a lightspeed receiver with a static HID++
device with ID 7 attached to it to. It is used to configure the led on
the mat. For this reason I increased the max number of devices.

Signed-off-by: Filipe Laíns <lains@archlinux.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>