pbio/task: don't run task from cancel function

Since efda5d0 ("drv/bluetooth_stm32_bluenrg: strict ordering of BLE
tasks"), it is no longer safe to call pbio_task_run_once() anywhere but
in the main contiki process loop of Bluetooth drivers.

This could break canceling some Bluetooth operations (causing them to
hang), but this will need to be fixed in a different way. A FIXME
comment with some suggestions is added to remind us to fix this.

lib/pbio/src/task.c

@@ -47,14 +47,15 @@ bool pbio_task_run_once(pbio_task_t *task) {
 }
 
 /**
- * Cancels @p task and runs one iteration unless the task is already complete.
+ * Sets cancel flag for @p task.
  * @param [in]  task The task.
  */
 void pbio_task_cancel(pbio_task_t *task) {
-    if (task->status != PBIO_ERROR_AGAIN) {
-        return;
-    }
-
     task->cancel = true;
-    pbio_task_run_once(task);
+    // FIXME: need a way to poll the process that has the task queue to
+    // expedite the cancellation. e.g. if the task is not the first in the
+    // queue, it can just be removed from the queue. For btstack, we probably
+    // need to create a synthetic event. For others drivers, we can probably
+    // do something with contiki events. Since it is driver-specific, this
+    // means we probably need to add a cancel callback to the task struct.
 }