soundwire: sysfs: cleanup the logic for creating the dp0 sysfs attributes

There's no need to special-case the dp0 sysfs attributes, the
is_visible() callback in the attribute group can handle that for us, so
add that and add it to the attribute group list making the logic simpler
overall.

This is a step on the way to moving all of the sysfs attribute handling
into the default driver core attribute group logic so that the soundwire
core does not have to do any of it manually.

Cc: Vinod Koul <[e-mail]>
Cc: Bard Liao <[e-mail]>
Cc: Pierre-Louis Bossart <[e-mail]>
Cc: Sanyog Kale <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>
Reviewed-by: Dan Williams <[e-mail]>
Tested-By: Vijendar Mukunda <[e-mail]>
Link: https://lore.kernel.org/r/2024013029-budget-mulled-5b34@gregkh
Signed-off-by: Vinod Koul <[e-mail]>

drivers/soundwire/sysfs_slave.c

···
 	NULL,
 };
 
+static umode_t dp0_attr_visible(struct kobject *kobj, struct attribute *attr,
+			      int n)
+{
+	struct sdw_slave *slave = dev_to_sdw_dev(kobj_to_dev(kobj));
+
+	if (slave->prop.dp0_prop)
+		return attr->mode;
+	return 0;
+}
+
+static bool dp0_group_visible(struct kobject *kobj)
+{
+	struct sdw_slave *slave = dev_to_sdw_dev(kobj_to_dev(kobj));
+
+	if (slave->prop.dp0_prop)
+		return true;
+	return false;
+}
+DEFINE_SYSFS_GROUP_VISIBLE(dp0);
+
 /*
  * we don't use ATTRIBUTES_GROUP here since we want to add a subdirectory
  * for dp0-level properties
  */
 static const struct attribute_group dp0_group = {
 	.attrs = dp0_attrs,
+	.is_visible = SYSFS_GROUP_VISIBLE(dp0),
 	.name = "dp0",
 };
 
 static const struct attribute_group *slave_groups[] = {
 	&slave_attr_group,
 	&sdw_slave_dev_attr_group,
+	&dp0_group,
 	NULL,
 };
 
···
 	ret = devm_device_add_groups(&slave->dev, slave_groups);
 	if (ret < 0)
 		return ret;
-
-	if (slave->prop.dp0_prop) {
-		ret = devm_device_add_group(&slave->dev, &dp0_group);
-		if (ret < 0)
-			return ret;
-	}
 
 	if (slave->prop.source_ports || slave->prop.sink_ports) {
 		ret = sdw_slave_sysfs_dpn_init(slave);