sfc: Add support for sub-10G speeds

The SFC4000 has a separate MAC for use at sub-10G speeds.  Introduce
an efx_mac_operations structure with implementations for the two MACs.
Switch between the MACs as necessary.

PHY settings are independent of the MAC, so add get_settings() and
set_settings() to efx_phy_operations.  Also add macs field to indicate
which MACs the PHY is connected to.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>