hwmon: (adt7475) 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
  10055	   7032	      0	  17087	   42bf	drivers/hwmon/adt7475.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  10567	   6520	      0	  17087	   42bf	drivers/hwmon/adt7475.o

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