usb: fix up post load checks

Correct post load checks:
1. dev->setup_len == sizeof(dev->data_buf)
    seems fine, no need to fail migration
2. When state is DATA, passing index > len
   will cause memcpy with negative length,
   resulting in heap overflow

First of the issues was reported by dgilbert.

Reported-by: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
