usb-bus: introduce a wrapper function to check speed

In this way, we can check speed directly, don't need
call usb_device_attach(), which has other conditions,
such as checking the chardev is open.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
