s390/tape: 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
  11511	    656	     16	  12183	   2f97	drivers/s390/char/tape_core.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  11575	    592	     16	  12183	   2f97	drivers/s390/char/tape_core.o

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