[media] cx88: Fix possible leak in cx8802_probe()

In case allocation vb2_dma_sg_init_ctx() fails during cx8802_probe(), the
already allocated cx8802 device structure memory is not freed in the used
exit path. Thus adapt the cleanup handling accordingly. Detected by Coverity
CID 1260065.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>