net: stmmac: ensure phydev is attached to phylink for C37 AN

As the support for MAC-side SGMII C37 AN is added to pcs-xpcs, phydev
should be attached to phylink during driver's open(). So, we change the
condition to "Not C73 AN" instead.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>