of_mdio: fix phy interrupt passing

The of_mdiobus_register_phy() is not setting phy->irq thus causing
some drivers to incorrectly assume that the PHY does not have an
IRQ associated with it. Not only do some drivers report no IRQ
they do not install an interrupt handler for the PHY.

Simplify the code setting irq and set the phy->irq at the same
time so that we cover the following issues, which should cover
all the cases the code will find:

- Set phy->irq if node has irq property and mdio->irq is NULL
- Set phy->irq if node has no irq and mdio->irq is not NULL
- Leave phy->irq as PHY_POLL default if none of the above

This fixes the issue:
 net eth0: attached PHY 1 (IRQ -1) to driver Micrel KSZ8041RNLI

to the correct:
 net eth0: attached PHY 1 (IRQ 416) to driver Micrel KSZ8041RNLI

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>