ASoC: audio-graph-card: add missing const at graph_get_dai_id()

commit c152f849 ("ASoC: audio-graph-card: fix an use-after-free in
graph_get_dai_id()") fixups use-after-free issue,
but, it need to use "const" for reg. This patch adds it.

We will have below without this patch

LINUX/sound/soc/generic/audio-graph-card.c: In function 'graph_get_dai_id':
LINUX/sound/soc/generic/audio-graph-card.c:87:7: warning: assignment discards\
 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
   reg = of_get_property(node, "reg", NULL);

Fixes: c152f849 ("ASoC: audio-graph-card: fix an use-after-free in graph_get_dai_id()")
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Wen Yang <wen.yang99@zte.com.cn>
Link: https://lore.kernel.org/r/87sgrd43ja.wl-kuninori.morimoto.gx@renesas.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/generic/audio-graph-card.c

@@ -63,7 +63,7 @@ static int graph_get_dai_id(struct device_node *ep)
 	struct device_node *endpoint;
 	struct of_endpoint info;
 	int i, id;
-	u32 *reg;
+	const u32 *reg;
 	int ret;
 
 	/* use driver specified DAI ID if exist */