net/fsl-pq-mdio: coalesce multiple memory allocations into one

Take advantage of the new mdiobus_alloc_size() function to combine three
different memory allocations into one.  This also simplies the error
handling.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>