Bluetooth: Add support for custom event terminated commands

This patch adds support for having commands within HCI requests that do
not result in a command complete but some other event. This is at least
needed for some vendor specific commands to be issued in the
hdev->setup() procecure, but might also be useful for other commands.

The way that the support is implemented is by extending the skb control
buffer to have a field to indicate that the command is expected to
terminate with a special event. After sending the command each received
event can then be compared against this field through hdev->sent_cmd.

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