net/fsl: enable extended scanning in xgmac_mdio

Since we know the xgmac hardware always has a c45
compliant bus, let's try scanning for c22 capable
PHYs first. If we fail to find any, then it will
fall back to c45 automatically.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Calvin Johnson <calvin.johnson@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>