i40e: enable fdb add code, remove unused code

The original FDB code submission wasn't correct and the code
wasn't enabled.  This removes some dead code (can use the common kernel
code for fdb_del and fdb_dump) and correctly enables the fdb_add
function pointer.

The fdb_add functionality is important to i40e because it is needed
for a workaround to allow bridges to work correctly on the i40e
hardware.

Reported-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>