V4L/DVB (11025): soc-camera: configure drivers with a default format on open

Currently soc-camera doesn't set up any image format without an explicit S_FMT.
It seems this should be supported, since, for example, capture-example.c from
v4l2-apps by default doesn't issue an S_FMT. This patch configures a default
image format on open().

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