Input: psmouse - 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
  12850	    740	     12	  13602	   3522	drivers/input/mouse/psmouse-base.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  12914	    676	     12	  13602	   3522	drivers/input/mouse/psmouse-base.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/mouse/psmouse-base.c

@@ -101,7 +101,7 @@ static struct attribute *psmouse_attributes[] = {
 	NULL
 };
 
-static struct attribute_group psmouse_attribute_group = {
+static const struct attribute_group psmouse_attribute_group = {
 	.attrs	= psmouse_attributes,
 };