platform/chrome: cros_ec_sysfs: Modify error handling

When accessing a sysfs attribute, if the EC command fails, -EPROTO is
now returned instead of an error message as it is unlikely an app is
parsing the error message to do something meaningful.
Also, this patch makes use of cros_ec_cmd_xfer_status() instead of
cros_ec_cmd_xfer() so an error message is printed in the syslog.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Benson Leung <bleung@chromium.org>