Bluetooth: Add support for vendor specific diagnostic channel

Introduce hci_recv_diag function for HCI drivers to allow sending vendor
specific diagnostic messages into the Bluetooth core stack. The messages
are not processed, but they are forwarded to the monitor channel and can
be retrieved by user space diagnostic tools.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>