Only check for gateway port on 'gateway set' ops

* The only time we should be checking for gateway ports is when we are setting
  the gateway on the router. Moved that check to its appropriate place.

Fixes MN-1062
Signed-off-by: Joe Mills <joe@midokura.com>
Merge pull request #63 from JoeMido/MN-1062v2

Only check for gateway port on 'gateway set' ops