e1000: fix WARN_ON with mac-vlan

When adding more than 14 mac-vlan adapters on e1000 the driver
would fire a WARN_ON when adding the 15th.  The WARN_ON in this
case is completely un-necessary, as the code below the WARN_ON is
directly handling the value the WARN_ON triggered on.

CC: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>