extcon: unregister compat link on cleanup

Since extcon registers this compat link at device registration
(extcon_dev_register), we should probably remove them at deregistration/cleanup.

Cc: stable@vger.kernel.org
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
[myungjoo.ham@samsung.com: merge conflict resolved for stable tree (pre-3.6)]
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>