net/mlx5e: MACsec, block offload requests with encrypt off

Currently offloading MACsec with authentication only (encrypt
property set to off) is not supported, block such requests
when adding/updating a macsec device.

Fixes: 8ff0ac5b

 ("net/mlx5: Add MACsec offload Tx command support")
Signed-off-by: Emeel Hakim <ehakim@nvidia.com>
Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>