tcpm: return all non-zero errors
tcpm: return all non-zero errors

TCPM functions calls I2C read/write, which returns positive error codes
on stm32f0 based EC.

BRANCH=none
BUG=none
TEST=none

Change-Id: I8119074e7411e2fd88dd6ca696fc7b698d462623
Signed-off-by: Rong Chang <3a05493f7adfb36d203f663b3bc2ba30e9d6284f@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/273506
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>

md5555/ec,coreboot/chrome-ec,fourier49/BIZ_EC,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,md5555/ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,coreboot/chrome-ec,coreboot/chrome-ec,fourier49/BIZ_EC,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,fourier49/BIZ_EC