samus: initiailze boostin_voltage in extpower module
samus: initiailze boostin_voltage in extpower module

Fix complile bug, initialize local var boostin_voltage.

BUG=none
BRANCH=samus
TEST=make -j buildall

Change-Id: Ie178d3bfe164fc900b1c52a05412f75d5a090ddd
Signed-off-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/250820

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