sfc: Cleaned up struct tso_state fields

Squashed nested structures.

Renamed remaining_len to out_len, ifc.len to in_len, header_length to
header_len.

Moved ipv4_id into the group of output variables where it belongs.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>