net: mvmdio: orion_mdio_ready: remove manual poll

Replace manual poll of MVMDIO_SMI_READ_VALID with a call to
orion_mdio_wait_ready.  This ensures a consistent timeout,
eliminates a busy loop, and allows for use of interrupts on
systems that support them.

Signed-off-by: Leigh Brown <leigh@solinno.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>