mtd: docg3: Fix passing zero to 'PTR_ERR' warning in doc_probe_device

Fix a static code checker warning:
drivers/mtd/devices/docg3.c:1875
 doc_probe_device() warn: passing zero to 'ERR_PTR'

Fixes: ae9d4934 ("mtd: docg3: add multiple floor support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>

drivers/mtd/devices/docg3.c

@@ -1872,7 +1872,7 @@ doc_probe_device(struct docg3_cascade *cascade, int floor, struct device *dev)
 nomem2:
 	kfree(docg3);
 nomem1:
-	return ERR_PTR(ret);
+	return ret ? ERR_PTR(ret) : NULL;
 }
 
 /**