USB: Move runtime PM callbacks to usb_device_pm_ops

USB defines usb_device_type pointing to usb_device_pm_ops that
provides system-wide PM callbacks only and usb_bus_type pointing to
usb_bus_pm_ops that provides runtime PM callbacks only.  However,
the USB runtime PM callbacks may be defined in usb_device_pm_ops
which makes it possible to drop usb_bus_pm_ops and will allow us
to consolidate the handling of subsystems by the PM core code.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>