Bluetooth: Add initial skeleton for asynchronous HCI requests

This patch adds the initial definitions and functions for asynchronous
HCI requests. Asynchronous requests are essentially a group of HCI
commands together with an optional completion callback. The request is
tracked through the already existing command queue by having the
necessary context information as part of the control buffer of each skb.

The only information needed in the skb control buffer is a flag for
indicating that the skb is the start of a request as well as the
optional complete callback that should be used when the request is
complete (this will be found in the last skb of the request).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>