ALSA: pcm: constify attribute_group structures.

attribute_groups are not supposed to change at runtime. All functions
working with attribute_groups provided by <linux/device.h> work with const
attribute_group. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   9781	    240	      8	  10029	   272d	sound/core/pcm.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   9813	    176	      8	   9997	   270d	sound/core/pcm.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm.c

@@ -1067,7 +1067,7 @@ static struct attribute *pcm_dev_attrs[] = {
 	NULL
 };
 
-static struct attribute_group pcm_dev_attr_group = {
+static const struct attribute_group pcm_dev_attr_group = {
 	.attrs	= pcm_dev_attrs,
 };