sfc: only advertise TX timestamping if we have the license for it

We check the license for TX hardware timestamping capability.
The PTP probe will have enabled PTP sync events from the adapter.  If
 later, at TX queue init, it turns out we do not have the license, we
 don't need the sync events either.

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>