ryu: Only wake PD task if the task has started
ryu: Only wake PD task if the task has started

On VBUS event, we should only wake the PD task if task_start() is
already called.

BRANCH=None
BUG=None
TEST=Repeatedly do sysjump while VBUS is toggling. Doesn't see crash
anymore.

Change-Id: Ie37e248b758e81e5fb9b7c1fd89fbff39f036086
Signed-off-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@google.com>
Reviewed-on: https://chromium-review.googlesource.com/242126
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Commit-Queue: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Tested-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>

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