ehci: add sanity check for maxframes

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20170703111549.10924-1-kraxel@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
