extcon: Fix build warning for extcon_unregister_notifier comment

Give the line the asterisk it wanted to fix the build warning.

[Build warning message]
Building with W=1 reports:

  CC      drivers/extcon/devres.o
drivers/extcon/devres.c:208: warning: bad line:
	- Resource-managed extcon_unregister_notifier()

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
[cw00.choi: Edit the patch subject and description]
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>