common/usbc_ppc: Fix potential illegal memory access.
common/usbc_ppc: Fix potential illegal memory access.

Recent coverity scan indicated a potential illegal memory access in
ppc_enter_low_power_mode().  This patch fixes it.

BUG=none
BRANCH=none
TEST=Compiled

Change-Id: I0df1ca23340cd4466f8e71349b89ca1ab68aadbf
Signed-off-by: Tim Wawrzynczak <86444811eee8c2cb29e29a251d7a069e64ccc665@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1574099
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
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