datapath: Additional fixes for datapath device renaming.

datapath: Additional fixes for datapath device renaming.

Commit c874dc6d6b "secchan: Fix behavior when a network device is renamed."
fixed a crash in the datapath when network devices within a datapath were
renamed.  However, this missed the case where the device that was renamed
was a datapath's internal port: these devices have their br_port members
set to NULL, so we have to determine that they belong to a datapath another
way.  This commit does so.

This commit also changes the initialization order in dp_dev_create().
Otherwise, dp_device_event() will dereference null when it is called via
register_netdevice(), because the newly created device is a datapath device
but its members are not yet initialized.