extcon: Fix missing locking when [un]registering notifiers

Since extcon.c is using raw_notifiers it must protect the notifier
list itself when [un]registering notifiers to avoid the list changing while
extcon_update_state is walking the list (through raw_notifier_call_chain).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[cw00.choi: Apply this patch to extcon.c driver instead of old extcon-class.c]
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>