USB: Export usb_wakeup_enabled_descendants()

In (e583d9db USB: global suspend and remote wakeup don't mix) we
introduced wakeup_enabled_descendants() as a static function.  We'd
like to use this function in USB controller drivers to know if we
should keep the controller on during suspend time, since doing so has
a power impact.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>