[media] v4l: soc-camera: fix multiple simultaneous user case

A recent patch has introduced a regression, whereby a second open of an
soc-camera video device breaks the running capture. This patch fixes this bug
by guaranteeing, that video buffers get initialised only during the first open
of the device node.

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