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: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
diff --git a/drivers/pci/pci-stub.c b/drivers/pci/pci-stub.c
index f7b68ca..4c0336b 100644
--- a/drivers/pci/pci-stub.c
+++ b/drivers/pci/pci-stub.c
@@ -47,6 +47,10 @@
 	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, ","))) {