ehci: adaptive wakeup rate.

Adapt the frame timer sleeps according to the actual needs.  With the
periodic schedule being active we'll have to wakeup 1000 times per
second and go check for work.  In case only the async schedule is active
we can be more lazy though.  When idle ehci will increate the sleep time
step by step, so qemu has to wake up less frequently.  When we'll see
transactions on the bus or the guest fiddles with the schedule
enable/disable bits we'll return to a 1000 Hz wakeup rate and full
speed.  With both schedules disabled we stop wakeups altogether.

This patch also drops the freq property (configures wakeup rate
manually) which is obsoleted by this patch.

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