sfc: Implement firmware-assisted TSO for EF10

Segmentation remains in the driver, but we generate option descriptors
describing the required packet editing rather than making our own
copies.

Reduce tso_state::ipv4_id to 16 bits, so it doesn't overflow into the
TCP_FLAGS field of the option descriptor.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>