platform/chrome: cros_ec_proto: return -EPROTO if empty payload

cros_ec_wait_until_complete() sends EC_CMD_GET_COMMS_STATUS which expects
to receive sizeof(struct ec_response_get_comms_status) from
cros_ec_xfer_command().

Return -EPROTO if cros_ec_xfer_command() returns 0.

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220718050914.2267370-11-tzungbi@kernel.org