vlan: set sysfs device_type to 'vlan'

Sets the sysfs device_type to 'vlan' for udev. This makes it easier for
applications that query network information via udev to identify vlans
instead of using strrchr().

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Signed-off-by: David S. Miller <davem@davemloft.net>