IB/mlx5: Set source mac address in FTE

Set the source mac address in the FTE when L2 specification
is provided.

Fixes: 038d2ef8 ('IB/mlx5: Add flow steering support')
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>