extcon: Initialize blocking notifier while registering

This change initializes the blocking notifier while registering
a new extcon device. Without this a warning is observed when
CONFIG_LOCKDEP is enabled since it is not as per lockdep
annotation, as the driver is trying to register a non-static key.

Change-Id: Ibb20fe11c4622ffd92f02508ffae5a4fb5625019
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>