static int unimac_mdio_write(struct mii_bus *bus, int phy_id,
			     int reg, u16 val)
{
	struct unimac_mdio_priv *priv = bus->priv;
	u32 cmd;

	/* Prepare the write operation */
	cmd = MDIO_WR | (phy_id << MDIO_PMD_SHIFT) |
		(reg << MDIO_REG_SHIFT) | (0xffff & val);
	unimac_mdio_writel(priv, cmd, MDIO_CMD);

	unimac_mdio_start(priv);

	return priv->wait_func(priv->wait_func_data);
}

The product dereferences a pointer that it expects to be valid but is NULL.