extcon: Remove potential problem when calling extcon_register_notifier()

[ Upstream commit 01b4c9a1ae07a25d208cad0da7dd288007a22984 ]

This patch removes the potential problem of extcon_register_notifier()
when edev parameter is NULL. When edev is NULL, this function returns
the first extcon device which includes the sepecific external connector
of second paramter. But, it don't guarantee the same operation in all cases.
To remove this confusion and potential problem, this patch fixes it.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>