of: property: use unsigned int return on of_graph_get_endpoint_count()

Because of of_graph_get_endpoint_count() doesn't report error,
just return count of endpoint, the return type should be unsigned.
Tidyup it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/87plxbcvzb.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Rob Herring <robh@kernel.org>