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

net/bridge/br_sysfs_br.c

@@ -865,7 +865,7 @@ static struct attribute *bridge_attrs[] = {
 	NULL
 };
 
-static struct attribute_group bridge_group = {
+static const struct attribute_group bridge_group = {
 	.name = SYSFS_BRIDGE_ATTR,
 	.attrs = bridge_attrs,
 };