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>