net: vlan: 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 vlan_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: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/8021q/vlan_dev.c

@@ -537,7 +537,7 @@ static const struct header_ops vlan_passthru_header_ops = {
 	.parse_protocol = vlan_parse_protocol,
 };
 
-static struct device_type vlan_type = {
+static const struct device_type vlan_type = {
 	.name	= "vlan",
 };