Bluetooth: Remove unneeded bt_cb(skb)->channel variable

The bt_cb(skb)->channel was only needed to make hci_send_to_sock() be
used for HCI raw and control sockets. Since they have now separate sending
functions this is no longer needed.

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