drivers: ethernet: ksz8081: fix compiling error about missing variable

The missing variable "config" in phy_mc_ksz8081_cfg_link() maybe
used by the macro "USING_INTERRUPT_GPIO", add the variable to avoid
compiling errors.

fixes: zephyrproject-rtos#95492

Signed-off-by: Tony Han <tony.han@microchip.com>

Author: TonyHan11

drivers/ethernet/phy/phy_microchip_ksz8081.c

@@ -474,6 +474,7 @@ static int phy_mc_ksz8081_reset(const struct device *dev)
 static int phy_mc_ksz8081_cfg_link(const struct device *dev, enum phy_link_speed speeds,
 				   enum phy_cfg_link_flag flags)
 {
+	__maybe_unused const struct mc_ksz8081_config *config = dev->config;
 	struct mc_ksz8081_data *data = dev->data;
 	int ret;