sfc: only enable TX timestamping if the adapter is licensed for it

If we try to enable the feature and do not have the license for it, the
 MCPU will refuse and fail our TX queue init.

Signed-off-by: Martin Habets <mhabets@solarflare.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>