drivers: fixed return code bug in eth_stm32_hal_set_config()

The function eth_stm32_hal_set_config() located in
drivers/ethernet/eth_stm32_hal.c always returns -ENOTSUP,
even if everything is fine. This commit fixes the return statement
so that the real result (ret) will be returned.

Signed-off-by: Bernd Weiberg <bernd.weiberg@siemens.com>