of_mdio: Fix broken PHY IRQ in case of probe deferral

If an Ethernet PHY is initialized before the interrupt controller it is
connected to, a message like the following is printed:

    irq: no irq domain found for /interrupt-controller@e61c0000 !

However, the actual error is ignored, leading to a non-functional (POLL)
PHY interrupt later:

    Micrel KSZ8041RNLI ee700000.ethernet-ffffffff:01: attached PHY driver [Micrel KSZ8041RNLI] (mii_bus:phy_addr=ee700000.ethernet-ffffffff:01, irq=POLL)

Depending on whether the PHY driver will fall back to polling, Ethernet
may or may not work.

To fix this:
  1. Switch of_mdiobus_register_phy() from irq_of_parse_and_map() to
     of_irq_get().
     Unlike the former, the latter returns -EPROBE_DEFER if the
     interrupt controller is not yet available, so this condition can be
     detected.
     Other errors are handled the same as before, i.e. use the passed
     mdio->irq[addr] as interrupt.
  2. Propagate and handle errors from of_mdiobus_register_phy() and
     of_mdiobus_register_device().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>