plankton: modify request_voltage after redundant 5V-PDO removal
plankton: modify request_voltage after redundant 5V-PDO removal

After removing redundant 5V source PDO for plankton, the index of
request_voltage need to minus 1.

BUG=chrome-os-partner:33737
BRANCH=samus
TEST=make BOARD=plankton, and test 5V/12V/20V buttons on plankton
board

Change-Id: I82f3f8ef72ce54221fb31027b10ff428f2440c9f
Reviewed-on: https://chromium-review.googlesource.com/229255
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Reviewed-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Commit-Queue: Pin-chih Lin <cb9a90359cbc928715293290f680153e174aed8c@chromium.org>
Tested-by: Pin-chih Lin <cb9a90359cbc928715293290f680153e174aed8c@chromium.org>
(cherry picked from commit f2aeae1efdd82439154d2d811d4e5e47a258c2b8)
Reviewed-on: https://chromium-review.googlesource.com/230046
Commit-Queue: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Tested-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>

thehobn/ec,thehobn/ec,thehobn/ec,thehobn/ec