net: phy: fixed_phy: Fix NULL vs IS_ERR() checking in __fixed_phy_register

The fixed_phy_get_gpiod function() returns NULL, it doesn't return error
pointers, using NULL checking to fix this.i

Fixes: 5468e82f ("net: phy: fixed-phy: Drop GPIO from fixed_phy_add()")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Link: https://lore.kernel.org/r/20211224021500.10362-1-linmq006@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/fixed_phy.c

@@ -239,8 +239,8 @@ static struct phy_device *__fixed_phy_register(unsigned int irq,
 	/* Check if we have a GPIO associated with this fixed phy */
 	if (!gpiod) {
 		gpiod = fixed_phy_get_gpiod(np);
-		if (IS_ERR(gpiod))
-			return ERR_CAST(gpiod);
+		if (!gpiod)
+			return ERR_PTR(-EINVAL);
 	}
 
 	/* Get the next available PHY address, up to PHY_MAX_ADDR */