HID: logitech-dj: rely on hid groups to separate receivers from dj devices

Several benefits here:
- we can drop the macro is_dj_device: I never been really conviced by
  this macro as we could fall into a null pointer anytime. Anyway time
  showed that this never happened.
- we can simplify the hid driver logitech-djdevice, and make it aware
  of any new receiver VID/PID.
- we can use the Wireless PID of the DJ device as the product id of the
  hid device, this way the sysfs will differentiate between different
  DJ devices.

Signed-off-by: Benjamin Tisssoires <benjamin.tissoires@redhat.com>
Tested-by: Andrew de los Reyes <adlr@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>