media: cx231xx: only unregister successfully registered i2c adapters

This prevents potentially scary debug messages from the i2c core.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>