hwmon: (adc128d818) constify attribute_group structures.

hwmon: (adc128d818) 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
   2304	   2936	      0	   5240	   1478	drivers/hwmon/adc128d818.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   2344	   2872	      0	   5216	   1460	drivers/hwmon/adc128d818.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>