[media] V4L: soc-camera: explicitly require V4L2_BUF_TYPE_VIDEO_CAPTURE

The soc-camera core accesses the "pix" member of the struct v4l2_format::fmt
union, which is only valid for V4L2_BUF_TYPE_VIDEO_CAPTURE streams. This
patch adds explicit checks for this to {g,s,try}_fmt methods.

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