usb-ohci: td.cbp incorrectly updated near page end

The current code that updates the cbp value after a transfer looks like this:
td.cbp += ret;
if ((td.cbp & 0xfff) + ret > 0xfff) {
	<handle page overflow>
because the 'ret' value is effectively added twice the check may fire too early
when the overflow hasn't happened yet.

Below is one of the possible changes that correct the behavior:
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb-ohci.c

@@ -1025,10 +1025,10 @@ static int ohci_service_td(OHCIState *ohci, struct ohci_ed *ed)
         if (ret == len) {
             td.cbp = 0;
         } else {
-            td.cbp += ret;
             if ((td.cbp & 0xfff) + ret > 0xfff) {
-                td.cbp &= 0xfff;
-                td.cbp |= td.be & ~0xfff;
+                td.cbp = (td.be & ~0xfff) + ((td.cbp + ret) & 0xfff);
+            } else {
+                td.cbp += ret;
             }
         }
         td.flags |= OHCI_TD_T1;