ALSA: pcm: use DEVICE_ATTR_RO macro

Use DEVICE_ATTR_RO() helper instead of plain DEVICE_ATTR(),
which makes the code a bit shorter and easier to read.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20210524120007.39728-1-yuehaibing@huawei.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm.c

@@ -1004,7 +1004,7 @@ void snd_pcm_detach_substream(struct snd_pcm_substream *substream)
 	substream->pstr->substream_opened--;
 }
 
-static ssize_t show_pcm_class(struct device *dev,
+static ssize_t pcm_class_show(struct device *dev,
 			      struct device_attribute *attr, char *buf)
 {
 	struct snd_pcm_str *pstr = container_of(dev, struct snd_pcm_str, dev);
@@ -1024,7 +1024,7 @@ static ssize_t show_pcm_class(struct device *dev,
 	return sprintf(buf, "%s\n", str);
 }
 
-static DEVICE_ATTR(pcm_class, 0444, show_pcm_class, NULL);
+static DEVICE_ATTR_RO(pcm_class);
 static struct attribute *pcm_dev_attrs[] = {
 	&dev_attr_pcm_class.attr,
 	NULL