platform/chrome: cros_ec_proto: Do not export cros_ec_cmd_xfer()

Now that all the remaining users of cros_ec_cmd_xfer() has been removed,
make this function private to the cros_ec_proto module.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>

drivers/platform/chrome/cros_ec_proto.c

@@ -496,8 +496,8 @@ EXPORT_SYMBOL(cros_ec_query_all);
  *
  * Return: 0 on success or negative error code.
  */
-int cros_ec_cmd_xfer(struct cros_ec_device *ec_dev,
-		     struct cros_ec_command *msg)
+static int cros_ec_cmd_xfer(struct cros_ec_device *ec_dev,
+			    struct cros_ec_command *msg)
 {
 	int ret;
 
@@ -541,7 +541,6 @@ int cros_ec_cmd_xfer(struct cros_ec_device *ec_dev,
 
 	return ret;
 }
-EXPORT_SYMBOL(cros_ec_cmd_xfer);
 
 /**
  * cros_ec_cmd_xfer_status() - Send a command to the ChromeOS EC.