extcon: Fix compile time warning

This patch fixes below compilation warning:-
drivers/extcon/extcon.c: In function extcon_register_notifier:
drivers/extcon/extcon.c:455:6: warning: idx may be used uninitialized in this function [-Wmaybe-uninitialized]
   if (idx >= 0) {
Signed-off-by: NVaneet Narang <v.narang@samsung.com>
Signed-off-by: NManinder Singh <maninder1.s@samsung.com>
[cw00.choi : Modify the patch title using the a captical letter for first char]
Signed-off-by: NChanwoo Choi <cw00.choi@samsung.com>

drivers/extcon/extcon.c

@@ -391,7 +391,7 @@ int extcon_register_notifier(struct extcon_dev *edev, unsigned int id,
 			     struct notifier_block *nb)
 {
 	unsigned long flags;
-	int ret, idx;
+	int ret, idx = -EINVAL;
 
 	if (!nb)
 		return -EINVAL;