Bluetooth: Refactor HCI request variables into own struct

In order to shrink the size of bt_skb_cb, this patch moves the HCI
request related variables into their own req_ctrl struct. Additionall
the L2CAP and HCI request structs are placed inside the same union since
they will never be used at the same time for the same skb.

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