platform/x86: eeepc-laptop: constify platform_attribute_group

File size before:
   text	   data	    bss	    dec	    hex	filename
   9934	   1136	      2	  11072	   2b40	drivers/platform/x86/eeepc-laptop.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   9998	   1072	      2	  11072	   2b40	drivers/platform/x86/eeepc-laptop.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>