IOS: Change devices to always return IPCCommandResult

This commit changes devices to always return IPCCommandResult rather
than just a return code for Open() and Close() in order to be able
to better emulate reply timing.

In hindsight, I should have considered we would want to emulate
timing when I cleaned up the device interface, but alas.
This rectifies that mistake.

Source/Core/Core/IOS/DeviceStub.h

@@ -21,7 +21,7 @@ class Stub final : public Device
 public:
   // Inherit the constructor from the Device class, since we don't need to do anything special.
   using Device::Device;
-  ReturnCode Open(const OpenRequest& request) override;
+  IPCCommandResult Open(const OpenRequest& request) override;
   IPCCommandResult IOCtl(const IOCtlRequest& request) override;
   IPCCommandResult IOCtlV(const IOCtlVRequest& request) override;
 };