xhci iso: allow for some latency

Allow the scheduled transfer time be a bit behind, to
compensate for latencies.  Without this xhci will wait
way to often for the mfindex wraparound, assuming the
scheduled time is in the future just because qemu is
a bit behind in processing the iso transfer requests.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-xhci.c

@@ -1977,7 +1977,7 @@ static void xhci_calc_iso_kick(XHCIState *xhci, XHCITransfer *xfer,
         xfer->mfindex_kick = ((xfer->trbs[0].control >> TRB_TR_FRAMEID_SHIFT)
                               & TRB_TR_FRAMEID_MASK) << 3;
         xfer->mfindex_kick |= mfindex & ~0x3fff;
-        if (xfer->mfindex_kick < mfindex) {
+        if (xfer->mfindex_kick + 0x100 < mfindex) {
             xfer->mfindex_kick += 0x4000;
         }
     }