drivers: ethernet: stm32: Add missing net_if_set_link_addr() call

When setting the MAC address, the ethernet driver has to call
net_if_set_link_addr() with the updated address. This was missing and is
added now.
See e.g.
https://github.com/zephyrproject-rtos/zephyr/pull/28874



Signed-off-by: Christian Taedcke <christian.taedcke@lemonbeat.com>