ofproto: Use netdev's 'change_seq' to reduce port modification check.

This commit uses the 'change_seq' in 'struct netdev' to determine
whether to update the 'ofport'.  This helps eliminate unnecessary
update.

In the experiment of configuring 5K internal port, there is neither
observable configuration overhead nor additional cpu consumption
after configuration finishes.  When one internal port state is
flapped every 0.3 second, this commit reduces the cpu utilization
of ovs-vswitchd thread from 40 to 12.

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