extcon: Fix attached value returned by is_extcon_changed

is_extcon_changed should only check the idx-th bit of new, not
the entirety of new when setting attached.

This fixes extcon sending notifications that a cable was inserted when
it gets removed while another cable is still connected.

Cc: stable@vger.kernel.org
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>