twinkie: turn on/off VCONN INA
twinkie: turn on/off VCONN INA

the VCONN INA is off by default to avoid a leakage path on CC2.
Turn it on when asking for a VCONN measurement ("twinkie vconn"),
then off aftwerwards.

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

BRANCH=none
BUG=none
TEST=On the Twinkie command-line, "tw vconn" with the Twinkie interposed
between a Samus and a DingDong.

Change-Id: I8cd78b285512644af0824a44c735585b684fee66
Reviewed-on: https://chromium-review.googlesource.com/239212
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Tested-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Reviewed-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>
Commit-Queue: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,eatbyte/chromium-ec,coreboot/chrome-ec,eatbyte/chromium-ec,mtk09422/chromiumos-platform-ec,fourier49/BIZ_EC,coreboot/chrome-ec,coreboot/chrome-ec,mtk09422/chromiumos-platform-ec,fourier49/BZ_DEV_EC,coreboot/chrome-ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BIZ_EC,eatbyte/chromium-ec,md5555/ec,mtk09422/chromiumos-platform-ec,eatbyte/chromium-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,fourier49/BZ_DEV_EC,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,coreboot/chrome-ec,fourier49/BIZ_EC,akappy7/ChromeOS_EC_LED_Diagnostics,mtk09422/chromiumos-platform-ec,md5555/ec