[media] si2168: Fix i2c_add_mux_adapter return value

In case of failure the return value was always 0. Return proper
error code (ENODEV) instead.

Signed-off-by: Luis Alves <ljalvs@gmail.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>