[media] cx23885: do not unregister demod I2C client twice on error

Demod I2C client should be NULL after demod is unregistered on error
path, otherwise it will be unregistered again when driver is unload.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>