net: dwc_eth_qos: Add eqos_get_enetaddr callback for stm32

Add .eqos_get_enetaddr callback defined as eqos_null_ops() to avoid
illegal access.

Fixes: a6242514 ("net: dwc_eth_qos: introduce eqos hook eqos_get_enetaddr")

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>