Bluetooth: Rename bt_cb()->req into bt_cb()->hci

The SKB context buffer for HCI request is really not just for requests,
information in their are preserved for the whole HCI layer. So it makes
more sense to actually rename it into bt_cb()->hci and also call it then
struct hci_ctrl.

In addition that allows moving the decoded opcode for outgoing packets
into that struct. So far it was just consuming valuable space from the
main shared items. And opcode are not valid for L2CAP packets.

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