net: 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
   9968	   3168	     16	  13152	   3360	net/core/net-sysfs.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  10160	   2976	     16	  13152	   3360	net/core/net-sysfs.o

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