net: bridge: constify attribute_group structures.

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

File size before:
   text	   data	    bss	    dec	    hex	filename
   2645	    896	      0	   3541	    dd5	net/bridge/br_sysfs_br.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   2701	    832	      0	   3533	    dcd	net/bridge/br_sysfs_br.o

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