Input: ads7846 - 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
   9086	    624	      0	   9710	   25ee	drivers/input/touchscreen/ads7846.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   9198	    496	      0	   9694	   25de	drivers/input/touchscreen/ads7846.o

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