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>