[media] via-camera: disable RGB mode

The RGB mode does not work correctly. It captures fine at 640x480
but whenever the scaling engine is used to produce another resolution,
color corruption occurs (lots of erroneous pink and green).

It is not clear how the scaling engine is supposed to work and how
it knows which pixel format it is dealing with. Work around this
problem by disabling RGB support. YUYV scaling works just fine.

Test case:

	gst-launch v4l2src ! video/x-raw-rgb,bpp=16,width=320,height=240 ! \
	ffmpegcolorspace ! xvimagesink

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>