V4L/DVB (10081): pxa-camera: call try_fmt() camera device method with correct pixel format

With the introduction of the format conversion support in soc-camera, we now
also have to take care to pass the correct pixel format to the camera driver
when calling its try_fmt() method.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>