nvdimm: constify device_type structures

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 declaration of device_type structure.

File size before:
  text	   data	    bss	    dec	    hex	filename
  19278	   3199	     16	  22493	   57dd	nvdimm/namespace_devs.o

File size after:
  text	   data	    bss	    dec	    hex	filename
  19929	   3160	     16	  23105	   5a41	nvdimm/namespace_devs.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/nvdimm/namespace_devs.c

@@ -52,17 +52,17 @@ static void namespace_blk_release(struct device *dev)
 	kfree(nsblk);
 }
 
-static struct device_type namespace_io_device_type = {
+static const struct device_type namespace_io_device_type = {
 	.name = "nd_namespace_io",
 	.release = namespace_io_release,
 };
 
-static struct device_type namespace_pmem_device_type = {
+static const struct device_type namespace_pmem_device_type = {
 	.name = "nd_namespace_pmem",
 	.release = namespace_pmem_release,
 };
 
-static struct device_type namespace_blk_device_type = {
+static const struct device_type namespace_blk_device_type = {
 	.name = "nd_namespace_blk",
 	.release = namespace_blk_release,
 };