Bluetooth: Keep a copy of each HID device's report descriptor

The report descriptor is read by user space (via the Service
Discovery Protocol), so it is only available during the ioctl
to connect. However, the HID probe function that needs the
descriptor might not be called until a specific module is
loaded. Keep a copy of the descriptor so it is available for
later use.

Signed-off-by: Michael Poole <mdpoole@troilus.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>