gpex: Fix type checking function name

This looks like a copy/paste mistake: the instance type checking
macro for TYPE_GPEX_ROOT_DEVICE was named MCH_PCI_DEVICE.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200902224311.1321159-2-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

include/hw/pci-host/gpex.h

@@ -32,7 +32,7 @@ DECLARE_INSTANCE_CHECKER(GPEXHost, GPEX_HOST,
 
 #define TYPE_GPEX_ROOT_DEVICE "gpex-root"
 typedef struct GPEXRootState GPEXRootState;
-DECLARE_INSTANCE_CHECKER(GPEXRootState, MCH_PCI_DEVICE,
+DECLARE_INSTANCE_CHECKER(GPEXRootState, GPEX_ROOT_DEVICE,
                          TYPE_GPEX_ROOT_DEVICE)
 
 #define GPEX_NUM_IRQS 4