[media] media: Fix a compiler warning in media_entity_pipeline_start()

Patch "[media] media: Print information on failed link validation" cause a
harmless compiler warning in printing a debug message. Fix this.

The type casting is done do ensure the type really is suitable for printing
as %u, as find_first_zero_bit() does return int on some architectures and
unsigned long on others.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>