net/mlx5e: TLS, refactor variable names

For symmetry, we rename mlx5e_tls_offload_context to
mlx5e_tls_offload_context_tx before we add mlx5e_tls_offload_context_rx.

Signed-off-by: Boris Pismenny <borisp@mellanox.com>
Reviewed-by: Aviad Yehezkel <aviadye@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>