nocturne: Discharge VBUS when stopping sourcing.
nocturne: Discharge VBUS when stopping sourcing.

When the source power path is turned off, we should discharge VBUS.

BUG=none
BRANCH=firmware-nocturne-10984.B
TEST=Flash nocturne, PR_SWAP, verify that VBUS is discharged instead of
floating during the swap.

Change-Id: I30cbb85cefc70ef161853758ece57b324eab0014
Signed-off-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1385445
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Tested-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec