extcon: Fix return value in extcon-class.c

Return the value obtained from the function extcon_register_interest
instead of -ENODEV.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>