drivers: eth: stm32: Fix invalid assertion comparison

The check for assertion on the "config_func" was added to
validate that the function pointer is valid. However, in
the code we are invoking the "config_func" and comparing
its output with NULL. This causes build failures with
CONFIG_ASSERT=1. Caused by PR-25393.

Tested on Nucleo F767Zi board.

Fixes #25427

Signed-off-by: Bilal Wasim <bilalwasim676@gmail.com>

drivers/ethernet/eth_stm32_hal.c

@@ -478,7 +478,7 @@ static void eth_iface_init(struct net_if *iface)
 		dev_data->iface = iface;
 
 		/* Now that the iface is setup, we are safe to enable IRQs. */
-		__ASSERT_NO_MSG(DEV_CFG(dev)->config_func() != NULL);
+		__ASSERT_NO_MSG(DEV_CFG(dev)->config_func != NULL);
 		DEV_CFG(dev)->config_func();
 	}