charger-manager: Fix a bug when it unregisters notifier block of extcon

This patch prevents NULL pointer error cauesed by unregistering
unregistered exton notifier block. At the probing time of charger manager,
it tries to remove extcon notifier block when it fails to initialize them.
It has to be applied for only registered one. Otherwise, it'd make kernel
panic. To make it work right, it checks extcon_specific_cable_nb's
extcon_dev node. If extcon cable notifier block was registered
successfully, it has proper extcon_dev pointer if not so it has NULL
pointer.

Signed-off-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>