tcpc: fix rx buffer overrun bug
tcpc: fix rx buffer overrun bug

Fix buffer overrun bug when retrieving a PD message. Bug was
introduced in CL:289005

BUG=chrome-os-partner:43482
BRANCH=none
TEST=tested on samus. plug and unplug zinger on both ports and
make sure PD MCU never crashes.

Change-Id: I9d2dec0cab07f389fd935d616ab7443da412d4bd
Signed-off-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/290417
Reviewed-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>

fourier49/BZ_DEV_EC,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,fourier49/BZ_DEV_EC