sfc: extend common GRO interface to support CHECKSUM_COMPLETE

EF100 will use CHECKSUM_COMPLETE, but will also make use of
 efx_rx_packet_gro(), thus needs to be able to pass the checksum value
 into that function.
Drivers for older NICs pass in a csum of 0 to get the old semantics (use
 the RX flags for CHECKSUM_UNNECESSARY marking).

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