xhci: Fix number of streams allocated when using streams

According to the xhci spec the total number of streams is
2 ^ (MaxPStreams + 1), and this is also how the Linux xhci driver
uses this field.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-xhci.c

@@ -1138,7 +1138,7 @@ static void xhci_reset_streams(XHCIEPContext *epctx)
 static void xhci_alloc_streams(XHCIEPContext *epctx, dma_addr_t base)
 {
     assert(epctx->pstreams == NULL);
-    epctx->nr_pstreams = 2 << epctx->max_pstreams;
+    epctx->nr_pstreams = 2 << (epctx->max_pstreams + 1);
     epctx->pstreams = xhci_alloc_stream_contexts(epctx->nr_pstreams, base);
 }