PCI: Skip id checking if no id is passed

Will get warning when pci stub driver is built-in kenel like:
pci-stub: invalid id string ""

So stop early if no id is passed.
Signed-off-by: NYinghai Lu <yinghai@kernel.org>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>

drivers/pci/pci-stub.c

@@ -47,6 +47,10 @@ static int __init pci_stub_init(void)
 	if (rc)
 		return rc;
 
+	/* no ids passed actually */
+	if (ids[0] == '\0')
+		return 0;
+
 	/* add ids specified in the module parameter */
 	p = ids;
 	while ((id = strsep(&p, ","))) {