xhci: Use 64bit writes to ERDP register.

At least some real-world XHCI controllers expect a 64bit write to the
ERDP register.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/hw/usb-xhci.c b/src/hw/usb-xhci.c
index dc98e5d..d0df6ee 100644
--- a/src/hw/usb-xhci.c
+++ b/src/hw/usb-xhci.c
@@ -379,6 +379,7 @@
         u32 addr = (u32)(&ir->erdp_low);
         u32 erdp = (u32)(evts->ring + nidx);
         pci_writel(addr, erdp);
+        pci_writel((u32)(&ir->erdp_high), 0);
     }
 }