i40e: fix the calculation of VFs mac addresses

num_mac should be increased only after the call to i40e_add_mac_filter().

Fixes: 5f527ba9

 ("i40e: Limit the number of MAC and VLAN addresses that can be added for VFs")
Signed-off-by: Zijie Pan <zijie.pan@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Tushar Dave <tushar.n.dave@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>