sfc: define inner/outer csum offload TXQ types

Nothing yet creates inner csum TXQs; just change all references to
 EFX_TXQ_TYPE_OFFLOAD to the new EFX_TXQ_TYPE_OUTER_CSUM.

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