Input: serio - 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
   6862	   1008	      4	   7874	   1ec2	drivers/input/serio/serio.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   6990	    880	      4	   7874	   1ec2	drivers/input/serio/serio.o

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