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: Vaneet Narang <v.narang@samsung.com>
Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
[cw00.choi : Modify the patch title using the a captical letter for first char]
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>