media: usb: cx231xx: convert to use i2c_new_client_device()

Move away from the deprecated API and make use of the fact that
unregistering devices is NULL- and ERR_PTR-safe.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>