net/mlx5e: Fix MACsec SA initialization routine

Currently as part of MACsec SA initialization routine
extended packet number (EPN) object attribute is always
being set without checking if EPN is actually enabled,
the above could lead to a NULL dereference.
Fix by adding such a check.

Fixes: 4411a6c0 ("net/mlx5e: Support MACsec offload extended packet number (EPN)")
Signed-off-by: Emeel Hakim <ehakim@nvidia.com>
Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>