Following coreboot's convention, return negative error codes from platform_i2c_transfer(). BUG=none TEST=emerge-asurada coreboot BRANCH=none Change-Id: I955b9aae11e20d75fac414d15714330e364dad2f Signed-off-by: Yu-Ping Wu <yupingso@chromium.org> Reviewed-on: https://review.coreboot.org/c/coreboot/+/59539 Tested-by: build bot (Jenkins) <no-reply@coreboot.org> Reviewed-by: Felix Held <felix-coreboot@felixheld.de>