platform/x86: alienware-wmi: constify attribute_group structures.

platform/x86: alienware-wmi: 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
   6932	   1016	     48	   7996	   1f3c	drivers/platform/x86/alienware-wmi.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   7060	    888	     48	   7996	   1f64	drivers/platform/x86/alienware-wmi.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>