USB: EHCI: elide I/O watchdog on AMD parts

This removes 10 timer wakeups per second. I'm running this patch for
a while now and haven't spotted any adverse effects.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>