[media] pxa_camera: make soc_mbus_xlate_by_fourcc() static

As warned by smatch:

drivers/media/platform/pxa_camera.c:283:39: warning: no previous prototype for 'soc_mbus_xlate_by_fourcc' [-Wmissing-prototypes]
 const struct soc_camera_format_xlate *soc_mbus_xlate_by_fourcc(
                                       ^~~~~~~~~~~~~~~~~~~~~~~~

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