[media] V4L: pxa-camera: try to force progressive video format

The pxa-camera driver only supports progressive video so far. Passing
down to the client the same format, as what the user has requested can
result in interlaced video, even if the client supports both. This
patch avoids such cases.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>