Bluetooth: Add hci_cmd_sync_cancel to public API

After transfer errors it makes sense to cancel an ongoing synchronous
command that cannot complete anymore. To permit this, export the old
hci_req_sync_cancel function as hci_cmd_sync_cancel in the API.

Signed-off-by: Benjamin Berg <bberg@redhat.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>