sfc: Do not pass non-TCP packets into GRO code

GRO can handle non-TCP packets and pass them up without coalescing,
but it has to do some extra work to parse the packet which we can
bypass using the hardware parse result.  (This condition yields a
false negative for TCP/IPv6 packets received by Falcon, but its
performance is already poor in that case due to lack of checksum
offload.)

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