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>

drivers/input/touchscreen/ads7846.c

@@ -499,7 +499,7 @@ static struct attribute *ads7846_attributes[] = {
 	NULL,
 };
 
-static struct attribute_group ads7846_attr_group = {
+static const struct attribute_group ads7846_attr_group = {
 	.attrs = ads7846_attributes,
 	.is_visible = ads7846_is_visible,
 };
@@ -599,7 +599,7 @@ static struct attribute *ads784x_attributes[] = {
 	NULL,
 };
 
-static struct attribute_group ads784x_attr_group = {
+static const struct attribute_group ads784x_attr_group = {
 	.attrs = ads784x_attributes,
 };