Bluetooth: Add __hci_cmd_sync() helper function

This patch adds a helper function for sending a single HCI command
waiting for its completion and then returning back the parameters in the
resulting command complete event (if there was one).

The implementation is very similar to that of hci_req_sync() except that
instead of invocing a callback for sending HCI commands the function
constructs and sends one itself and after being woken up picks the last
received event from hdev->recv_evt (if it matches the right criteria)
and returns it.

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