ar71xx: Fix mikrotik NAND compile problem

This Fixes the folowing compile error:
drivers/mtd/nand/rb91x_nand.c: In function 'rb91x_nand_remove':
drivers/mtd/nand/rb91x_nand.c:445:16: error: 'rbni' undeclared (first use in this function)
  nand_release(&rbni->chip);

Fixes: ce958dd88a ("kernel: Update kernel 4.14 to version 4.14.187")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/ar71xx/files/drivers/mtd/nand/rb91x_nand.c

@@ -442,7 +442,7 @@ static int rb91x_nand_remove(struct platform_device *pdev)
 {
 	struct rb91x_nand_info *info = platform_get_drvdata(pdev);
 
-	nand_release(&rbni->chip);
+	nand_release(&info->chip);
 
 	return 0;
 }