net: stmmac: set phy to use polling by default

mii_irq[] array is never initialized anywhere in the driver, thus mii_irq[]
will always equate to zero. So, for the case where the PHY does not have an
irq, we should use PHY_POLL for that situation.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Tested-by: Vince Bridgers <vbridger@altera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>