extcon: Fix return value in extcon_register_interest()

Propagate the value returned from extcon_find_cable_index()
instead of -ENODEV. For readability, -EINVAL is returned in place of
the variable.

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