net: dsa: do not use slave MII bus for fixed PHYs

Commit cd28a1a9 ("net: dsa: fully divert PHY reads/writes if
requested") introduced a check for particular PHYs that need to be
accessed using the slave MII bus created by DSA, but this check was too
inclusive. This would prevent fixed PHYs from being successfully
registered because those should not go through the slave MII bus created
by DSA.

Make sure we check that the PHY is not a fixed PHY to prevent that from
happening.

Fixes: cd28a1a9 ("net: dsa: fully divert PHY reads/writes if requested")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>