arm: stm32mp: cleanup test on eth_env_set_enetaddr result

Remove the unnecessary inversion on the eth_env_set_enetaddr() result which
only make complex the code of setup_mac_address() and display an invalid
value in the associated pr_err.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Marek Vasut <marex@denx.de>