sfc: basic MCDI mapping of 25/50/100G link speeds

Only handles direct speed setting, not autoneg, because the driver is
 still trying to pretend it uses the legacy ethtool API which doesn't
 have advertised/supported bits for 25/50/100G.

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