Kernel: Use a WaitQueue in PATAChannel

Instead of waking up repeatedly to check if a disk operation has
finished, use a WaitQueue and wake it up in the IRQ handler.

This simplifies the device driver a bit, and makes it more responsive
as well :^)

Kernel/Thread.h

@@ -322,6 +322,11 @@ public:
         return block<ConditionBlocker>(state_string, move(condition));
     }
 
+    void wait_on(WaitQueue& queue)
+    {
+        (void)block<WaitQueueBlocker>(queue);
+    }
+
     void unblock();
 
     // Tell this thread to unblock if needed,