ALSA: Create sysfs attribute files via groups

Instead of calling each time device_create_file(), create the groups
of sysfs attribute files at once in a normal way.  Add a new helper
function, snd_get_device(), to return the associated device object,
so that we can handle the sysfs addition locally.

Since the sysfs file addition is done differently now,
snd_add_device_sysfs_file() helper function is removed.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/hwdep.h

@@ -68,6 +68,7 @@ struct snd_hwdep {
 	wait_queue_head_t open_wait;
 	void *private_data;
 	void (*private_free) (struct snd_hwdep *hwdep);
+	const struct attribute_group **groups;
 
 	struct mutex open_mutex;
 	int used;			/* reference counter */