ehci: add freq + maxframes properties

Add properties for the wakeup rate and the max number of frames ehci
will process at once.

The wakeup rate defaults to 1000 which equals the usb frame rate.  This
can be reduced to make qemu wake up less often when ehci is active.

In case the wakeup rate is reduced or the ehci timer is delayed due to
latency issues elsewhere in qemu ehci will process multiple frames at
once.  The maxframes property specifies the upper limit for this.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
