sfc: Fix conditions for MDIO self-test

The MDIO self-test should not be run on boards without an MDIO PHY,
such as SFN5122F-R3 and later revisions.  It should also not try to
address a specific MMD in an MDIO clause 22 PHY.  Check the
mode_support field to decide which mode to use, if any.

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