[media] saa7115: Trust that V4L2 core will fill the mask

Instead of using V4L2_STD_ALL when no standard is detected,
trust that the maximum allowed standards are already filled by
the V4L2 core. It is better this way, as the bridge and/or the audio
decoder may have some extra restrictions to some video standards.

This also allow other devices like audio and tuners to contribute to
standards detection, when they support such feature.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>