s390/qeth: constify attribute_group structures.

s390/qeth: 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
   6763	   1216	      0	   7979	   1f2b	drivers/s390/net/qeth_l3_sys.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   7019	    960	      0	   7971	   1f2b	drivers/s390/net/qeth_l3_sys.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>