sfc: simplify RX datapath timestamping

sfc: simplify RX datapath timestamping

BugLink: https://bugs.launchpad.net/bugs/1836635
Use timestamp conversion function with correction to avoid duplicate
 correction handling.

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>
(cherry picked from commit c4f64fcc4d31e7f773cb4eec9d90c40ebb049c14)
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>