media: v4l2-fwnode: suppress a warning at OF parsing logic

smatch produce this warning:
	drivers/media/v4l2-core/v4l2-fwnode.c:76 v4l2_fwnode_endpoint_parse_csi_bus() error: buffer overflow 'array' 5 <= u16max

That's because, in thesis, the routine might have called with
some value at bus->num_data_lanes. That's not the current
case.

Yet, better to shut up this warning, and make the code more
reliable if some future changes might cause a bug.

While here, simplify the code a little bit by reading only
once from lanes-properties array.

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