net: ppp: constify the struct device_type usage

Since commit aed65af1 ("drivers: make device_type const"), the driver
core can properly handle constant struct device_type. Move the ppp_type
variable to be a constant structure as well, placing it into read-only
memory which can not be modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp/ppp_generic.c

@@ -1607,7 +1607,7 @@ static const struct net_device_ops ppp_netdev_ops = {
 	.ndo_fill_forward_path = ppp_fill_forward_path,
 };
 
-static struct device_type ppp_type = {
+static const struct device_type ppp_type = {
 	.name = "ppp",
 };