[media] soc-camera: properly fix camera probing races

The recently introduced host_lock causes lockdep warnings, besides, list
enumeration in scan_add_host() must be protected by holdint the list_lock.
OTOH, holding .video_lock in soc_camera_open() isn't enough to protect
the host during its building of the pipeline, because .video_lock is per
soc-camera device. If, e.g. more than one sensor can be attached to a host
and the user tries to open both device nodes simultaneously, host's .add()
method can be called simultaneously for both sensors. Fix these problems
by holding list_lock instead of .host_lock in scan_add_host() and taking
it shortly at the beginning of soc_camera_open(), and using .host_lock to
protect host's .add() and .remove() operations only.

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