net: phy: phy_device: Prevent nullptr exceptions on ISR

[ Upstream commit 61c81872

 ]

If phydev->irq is set unconditionally, check
for valid interrupt handler or fall back to polling mode to prevent
nullptr exceptions in interrupt service routine.

Signed-off-by: Andre Werner <andre.werner@systec-electronic.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240129135734.18975-2-andre.werner@systec-electronic.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>