ehci: fix overflow in frame timer code

In case the frame timer doesn't run for a while due to the host being
busy skipped_uframes can become big enough that UFRAME_TIMER_NS *
skipped_uframes overflows.  Which in turn throws off all subsequent
ehci frame timer calculations.

Reported-by: 李林 <8610_28@163.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20170515104543.32044-1-kraxel@redhat.com
