ospf6d: fix warning message when interfae disabled

ospf6d: fix warning message when interfae disabled

When removing an interface from an existing area,
the warning message we get is not correct

  interface r1-r2-eth0
   ipv6 address 2013:12::1/64
   ipv6 ospf6 dead-interval 4
   ipv6 ospf6 hello-interval 1
  !
  interface dummy
   ipv6 ospf6 dead-interval 4
   ipv6 ospf6 hello-interval 1
   ipv6 ospf6 network point-to-point
  !
  router ospf6
   ospf6 router-id 1.1.1.1
   interface r1-r2-eth0 area 0.0.0.0
  !

  r1(config-if)# router ospf6
  r1(config-ospf6)# no interface dummy area 0.0.0.0
  No such Area-ID: 0.0.0.0   <--- area 0.0.0.0 exists

This is fixing the error message

Signed-off-by: ckishimo <carles.kishimoto@gmail.com>