Bluetooth: Introduce new hci_req_add function

This function is analogous to hci_send_cmd() but instead of directly
queuing the command to hdev->cmd_q it adds it to the local queue of the
asynchronous HCI request being build (inside struct hci_request).

This is the main function used for building asynchronous requests and
there should be one or more calls to it between calls to hci_req_init
and hci_req_run.

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>