net/mlx5e: Fix macsec possible null dereference when updating MAC security entity (SecY)

Upon updating MAC security entity (SecY) in hw offload path, the macsec
security association (SA) initialization routine is called. In case of
extended packet number (epn) is enabled the salt and ssci attributes are
retrieved using the MACsec driver rx_sa context which is unavailable when
updating a SecY property such as encoding-sa hence the null dereference.
Fix by using the provided SA to set those attributes.

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>