When the enable bits for controller / async schedule / periodic schedule
change just make sure we kick the frame timer and let
ehci_advance_periodic_state and ehci_advance_async_state handle the
controller state changes.

This will make ehci set USBSTS_HALT when the controller shutdown is
actually done, once both schedules are in inactive state and the
USBSTS_PSS and USBSTS_ASS bits are clear.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>