media: rc: 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
  11605	    880	     20	  12505	   30d9	drivers/media/rc/rc-main.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  11797	    720	     20	  12537	   30f9	drivers/media/rc/rc-main.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>