[media] ov9640: use macro to request OmniVision OV9640 sensor private data

This cleanup patch creates macro to request OmniVision OV9640 private
data, which increases readability.

Signed-off-by: David Cohen <dacohen@gmail.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>