drivers: eth: stm32: Fix invalid assertion check

[bwasim]

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

[bwasim]

Build failure in shippable not seen under #25393, but seen under #25397.. Here's the log,

In file included from ../../../../../../include/kernel_includes.h:22,
                 from ../../../../../../include/kernel.h:17,
                 from /home/buildslave/src/github.com/zephyrproject-rtos/zephyr/drivers/ethernet/eth_stm32_hal.c:12:
/home/buildslave/src/github.com/zephyrproject-rtos/zephyr/drivers/ethernet/eth_stm32_hal.c: In function 'eth_iface_init':
/home/buildslave/src/github.com/zephyrproject-rtos/zephyr/drivers/ethernet/eth_stm32_hal_priv.h:48:3: error: void value not ignored as it ought to be
   48 |  ((const struct eth_stm32_hal_dev_cfg *)(dev)->config_info)
../../../../../../include/sys/__assert.h:86:9: note: in definition of macro '__ASSERT_NO_MSG'
   86 |   if (!(test)) {                                            \
      |         ^~~~
/home/buildslave/src/github.com/zephyrproject-rtos/zephyr/drivers/ethernet/eth_stm32_hal.c:481:19: note: in expansion of macro 'DEV_CFG'
  481 |   __ASSERT_NO_MSG(DEV_CFG(dev)->config_func() != NULL);
      |                   ^~~~~~~
[48/165] Building C object zephyr/arch/arch/arm/core/aarch32/cortex_m/CMakeFiles/arch__arm__core__aarch32__cortex_m.dir/scb.c.obj

[pfalcon]

@carlescufi: Can you please merge this, there're reports that it breaks CI.