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: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>