usb: async control xfer fixup

Need to clear p->result after copying setup data using usb_packet_copy()
because we'll reuse the USBPacket for the data transfer.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/hw/usb/core.c b/hw/usb/core.c
index 01a7622..c7e5bc0 100644
--- a/hw/usb/core.c
+++ b/hw/usb/core.c
@@ -107,6 +107,7 @@
     }
 
     usb_packet_copy(p, s->setup_buf, p->iov.size);
+    p->result = 0;
     s->setup_len   = (s->setup_buf[7] << 8) | s->setup_buf[6];
     s->setup_index = 0;