spring: Avoid picking up USB power from AP
spring: Avoid picking up USB power from AP

When powered device is removed, VBUS is still powered by the AP. We
should turn off power from AP and let it settle before we sense external
VBUS input.

BUG=chrome-os-partner:14319
TEST=Manual on Spring
BRANCH=none

Change-Id: Ie868a3df27e395c948ba8c6342e84764d182ce82
Signed-off-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/43779
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

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