netdev-vport: Do not update netdev when there is no config change.

netdev-vport: Do not update netdev when there is no config change.

When there is any update from ovsdb, ovs will call netdev_set_config()
for every vport.  Even though the change is not related to vport, the
current implementation will always increment the per-netdev sequence
number.  Subsequently this could cause even more unwanted effects,
e.g. the recreation of 'struct tnl_port' in ofproto level.

This commit fixes the issue by only updating the netdev when there
is actual configuration change.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>