net/mlx5e: Refactor duplicated code in mlx5e_ipsec_init_macs

ARP discovery code has same logic for RX and TX flows, but with
different source and destination fields. Instead of duplicating
same code in mlx5e_ipsec_init_macs, let's refactor.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>