r8169: fix crash if CONFIG_DEBUG_SHIRQ is enabled

If CONFIG_DEBUG_SHIRQ is enabled __free_irq() intentionally fires
a spurious interrupt. This interrupt causes a crash because
tp->dev->phydev is NULL at that time.

Fixes: 38caff5a ("r8169: handle all interrupt events in the hard irq handler")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>