net: fsl_pq_mdio: fix non tbi phy access

Since 952c5ca1

 (fsl_pq_mdio: Clean up tbi address configuration) .probe returns
-EBUSY when the "tbi-phy" node is missing. Fix this.

Cc: Andy Fleming <afleming@freescale.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>