media: imon: constify attribute_group structures

media: imon: 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
  18551	   2256	     77	  20884	   5194	drivers/media/rc/imon.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  18679	   2160	     77	  20916	   51b4	drivers/media/rc/imon.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>