ehci: fix overflow in frame timer code

In case the frame timer doesn't run for a while due to the host being
busy skipped_uframes can become big enough that UFRAME_TIMER_NS *
skipped_uframes overflows.  Which in turn throws off all subsequent
ehci frame timer calculations.

Reported-by: 李林 <8610_28@163.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20170515104543.32044-1-kraxel@redhat.com

hw/usb/hcd-ehci.c

@@ -2238,7 +2238,7 @@ static void ehci_frame_timer(void *opaque)
     int need_timer = 0;
     int64_t expire_time, t_now;
     uint64_t ns_elapsed;
-    int uframes, skipped_uframes;
+    uint64_t uframes, skipped_uframes;
     int i;
 
     t_now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);