zinger: do not cut output on up voltage transitions
zinger: do not cut output on up voltage transitions

Update according to the current PD standard, a monotonic transition
seems mandatory in all cases, so keep the voltage output enabled
when increasing the output voltage.

Signed-off-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

BRANCH=none
BUG=chrome-os-partner:28331
TEST=connect Zinger to Fruitpie and probe the VBUS voltage during a
transition.

Change-Id: I3c728cc0049ca41536efd4f075139626b7d371da
Reviewed-on: https://chromium-review.googlesource.com/202657
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Commit-Queue: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Tested-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

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