media: mc: Fix graph walk in media_pipeline_start

The graph walk tries to follow all links, even if they are not between
pads. This causes a crash with, e.g. a MEDIA_LNK_FL_ANCILLARY_LINK link.

Fix this by allowing the walk to proceed only for MEDIA_LNK_FL_DATA_LINK
links.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Fixes: ae219872 ("media: mc: entity: Rewrite media_pipeline_start()")