HID: hid-logitech-dj: fix off by one

There is a bug where a device with index 6 would write out of bounds in
the array of paired devices.
This patch fixes that problem.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Reviewed-by: Olivier Gay <ogay@logitech.com>
Signed-off-by: Nestor Lopez Casado <nlopezcasad@logitech.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>