video: fbdev: uvesafb: constify attribute_group structures.

attribute_group are not supposed to change at runtime. All functions
working with attribute_group 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
  15426	   4952	    187	  20565	   5055	drivers/video/fbdev/uvesafb.o

File size after adding 'const':
   text	   data	    bss	    dec	    hex	filename
  15490	   4888	    187	  20565	   5055	drivers/video/fbdev/uvesafb.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Cc: Michal Januszewski <spock@gentoo.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>