n64cart: fix return value check in n64cart_probe()

In case of error, the function devm_platform_ioremap_resource()
returns ERR_PTR() and never returns NULL. The NULL test in the
return value check should be replaced with IS_ERR().

Fixes: d9b2a2bb ("block: Add n64 cart driver")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NChaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20210909090608.2989716-1-yangyingliang@huawei.comSigned-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/n64cart.c

@@ -129,8 +129,8 @@ static int __init n64cart_probe(struct platform_device *pdev)
 	}
 
 	reg_base = devm_platform_ioremap_resource(pdev, 0);
-	if (!reg_base)
-		return -EINVAL;
+	if (IS_ERR(reg_base))
+		return PTR_ERR(reg_base);
 
 	disk = blk_alloc_disk(NUMA_NO_NODE);
 	if (!disk)