HID: logitech-hidpp: rework probe path for unifying devices

Unifying devices are different from others because they can probed
while not connected. So we need to talk to the receiver to get some
extra information like the device name and the serial.

Instead of having conditionals while attempting to read the device name
from HID++ 2.0, have a special init path for them.

Store the retrieved serial in hdev->uniq.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>