cros_ec: Complete the comment for cros_ec_read_batt_charge()

Add the missing 'Return' information.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/cros_ec.h b/include/cros_ec.h
index 2a77286..94c988a 100644
--- a/include/cros_ec.h
+++ b/include/cros_ec.h
@@ -648,6 +648,7 @@
  *
  * @dev: CROS-EC device
  * @chargep: Return battery-charge state as a percentage
+ * Return: 0 if OK, -ve on error
  */
 int cros_ec_read_batt_charge(struct udevice *dev, uint *chargep);