sfc: support separate PTP and general timestamping

Support MC_CMD_PTP_OUT_GET_TIMESTAMP_CORRECTIONS_V2.  Extract general
 timestamp corrections in addition to PTP corrections.  Apply receive
 timestamp corrections for general datapath receive timestamping, and
 correspondingly for transmit.

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