rbd: constify device_type structure

Declare device_type structure as const as it is only stored in the
type field of a device structure. This field is of type const, so add
const to the declaration of device_type structure.

File size before:
   text	   data	    bss	    dec	    hex	filename
  61546	  11610	    208	  73364	  11e94	drivers/block/rbd.o

File size after:
   text	   data	    bss	    dec	    hex	filename
  61610	  11578	    208	  73396	  11eb4	drivers/block/rbd.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>