pci: bridge control fixup

PCI_BRIDGE_CTL_DISCARD_STATUS (bit 10 in bridge control register)
is W1C so we should not make it writeable, otherwise the assert(!(wmask
& w1cmask)) in pci_default_write_config() is hit

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reported-by: Isaku Yamahata <yamahata@valinux.co.jp>
Tested-by: Isaku Yamahata <yamahata@valinux.co.jp>

hw/pci.c

@@ -641,7 +641,6 @@ static void pci_init_wmask_bridge(PCIDevice *d)
                  PCI_BRIDGE_CTL_FAST_BACK |
                  PCI_BRIDGE_CTL_DISCARD |
                  PCI_BRIDGE_CTL_SEC_DISCARD |
-                 PCI_BRIDGE_CTL_DISCARD_STATUS |
                  PCI_BRIDGE_CTL_DISCARD_SERR);
     /* Below does not do anything as we never set this bit, put here for
      * completeness. */