sfc: add function to determine which TX timestamping method to use

Use MC capability MC_CMD_GET_CAPABILITIES_V2_OUT_TX_MAC_TIMESTAMPING to
 detect whether the NIC supports timestamping packets sent out the main
 datapath.

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