net: cdc_ncm: constify attribute_group structures.

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

File size before:
   text	   data	    bss	    dec	    hex	filename
  13275	    928	      1	  14204	   377c	drivers/net/usb/cdc_ncm.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  13339	    864	      1	  14204	   377c	drivers/net/usb/cdc_ncm.o
Signed-off-by: NArvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/usb/cdc_ncm.c

@@ -367,7 +367,7 @@ static struct attribute *cdc_ncm_sysfs_attrs[] = {
 	NULL,
 };
 
-static struct attribute_group cdc_ncm_sysfs_attr_group = {
+static const struct attribute_group cdc_ncm_sysfs_attr_group = {
 	.name = "cdc_ncm",
 	.attrs = cdc_ncm_sysfs_attrs,
 };