net: stmmac: Log MAC address only once

Logging the MAC address on every if-up, is not really useful, and annoying when
there is no cable inserted and NetworkManager tries the ifup every 50 seconds.

Also change the log level from warning to info, as that is what it is.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>