[media] V4L: soc-camera: protect hosts during probing from overzealous user-space

If multiple clients are registered on a single camera host interface,
the user-space hot-plug software can try to access the one, that probed
first, before probing of the second one has completed. This can be
handled by individual host drivers, but it is even better to hold back
the user-space until all the probing on this host has completed. This
fixes a race on ecovec with two clients registered on the CEU1 host, which
otherwise triggers a BUG() in sh_mobile_ceu_remove_device().

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