tunnel: Recreate tunnel port only when the netdev status change.

On current master, the 'struct tnl_port' in tunnel module will be
recreated whenever the global connectivity sequence number changes
(e.g. when adding unrelated flow).  This is unnecessary and could
cause drop of tunnel packet if a lookup happens between the removal
and recreate.

This commit fixes the above issue by only checking the netdev's own
sequence number.

Found by code inspection.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>