kernel/readable_event: Add member function for enforcing a strict reset contract

svcResetSignal relies on the event instance to have already been
signaled before attempting to reset it. If this isn't the case, then an
error code has to be returned.

src/core/hle/kernel/readable_event.cpp

@@ -4,10 +4,10 @@
 
 #include <algorithm>
 #include "common/assert.h"
+#include "core/hle/kernel/errors.h"
 #include "core/hle/kernel/object.h"
 #include "core/hle/kernel/readable_event.h"
 #include "core/hle/kernel/thread.h"
-#include "core/hle/kernel/writable_event.h"
 
 namespace Kernel {
 
@@ -34,6 +34,16 @@ void ReadableEvent::Clear() {
     signaled = false;
 }
 
+ResultCode ReadableEvent::Reset() {
+    if (!signaled) {
+        return ERR_INVALID_STATE;
+    }
+
+    Clear();
+
+    return RESULT_SUCCESS;
+}
+
 void ReadableEvent::WakeupAllWaitingThreads() {
     WaitObject::WakeupAllWaitingThreads();