[media] soc-camera: don't dereference I2C client after it has been removed

i2c_unregister_device() frees the I2C client, so, dereferencing it
afterwards is a bug, that leads to Oopses.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>