phylib: introduce mdiobus_alloc_size()

Introduce function mdiobus_alloc_size() as an alternative to mdiobus_alloc().
Most callers of mdiobus_alloc() also allocate a private data structure, and
then manually point bus->priv to this object.  mdiobus_alloc_size()
combines the two operations into one, which simplifies memory management.

The original mdiobus_alloc() now just calls mdiobus_alloc_size(0).

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