V4L/DVB (13344): soc-camera: properly initialise the device object when reusing

Commit ef373189f62413803b7b816c972fc154c488cdc0 "fix use-after-free Oops,
resulting from a driver-core API change" fixed the Oops, but didn't correct
missing device object initialisation. This patch makes unloading and reloading
of soc-camera host- and client-drivers possible again.

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