hwmon: (nct7802) 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
   6161	   9400	      0	  15561	   3cc9	drivers/hwmon/nct7802.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   6465	   9080	      0	  15545	   3cb9	drivers/hwmon/nct7802.o

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