media: ov2740: Move fwnode_graph_get_next_endpoint() call up

If the bridge has not yet setup the fwnode-graph then
the fwnode_property_read_u32("clock-frequency") call will fail.

Move the fwnode_graph_get_next_endpoint() call to above reading
the clock-frequency.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>