media: cx18: properly report pixelformats

The MPEG device reported non-MPEG pixelformats, and the YUV device reported
the MPEG pixelformat as well.

Separate the two: either report MPEG or HM12/UYUV, not all three.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>