sfc: decouple TXQ type from label

Make it possible to have an arbitrary mapping from types to labels,
 because when we add inner-csum-offload TXQs there will no longer be a
 convenient nesting hierarchy of NIC types (EF10 will have inner-csum
 TXQs, while Siena will have HIGHPRI).
Correct a misleading comment on efx_hard_start_xmit().

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