block: swim3: make of_device_ids const.

of_device_ids are not supposed to change at runtime. All functions
working with of_device_ids provided by <linux/of.h> work with const
of_device_ids. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   8908	   1096	    624	  10628	   2984	drivers/block/swim3.o

File size after constify swim3_match:
   text	   data	    bss	    dec	    hex	filename
   9708	    296	    624	  10628	   2984	drivers/block/swim3.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@fb.com>