net: usb: lan78xx: lan78xx_phy_init(): use PHY_POLL instead of "0" if no IRQ is available

On most systems request for IRQ 0 will fail, phylib will print an error message
and fall back to polling. To fix this set the phydev->irq to PHY_POLL if no IRQ
is available.

Fixes: cc89c323 ("lan78xx: Use irq_domain for phy interrupt from USB Int. EP")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Sven Schuchmann <schuchmann@schleissheimer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>