enic: Bug Fix: Fix return values of enic_add/del_station_addr routines

Fix enic_add/del_station_addr routines to return appropriate error code when an invalid address is added or deleted.

Signed-off-by: Christian Benvenuti <benve@cisco.com>
Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: David Wang <dwang2@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>