sfc: Replace PHY MDIO test with an 'alive' test

SFC9000-family boards do not all use MDIO PHYs, so we need a different
test for PHY aliveness.

Introduce a PHY operation test_alive().  For PHYs attached to Falcon,
use a common implementation based on the existing PHY MDIO test.
For PHYs managed through MCDI, use the appropriate MCDI request.

Change test name in ethtool from 'core mdio' to 'phy alive'.

Rename test_results::mdio to phy_alive and test_results::phy to phy_ext.

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