net: stmmac: don't use netdev_[dbg, info, ..] before net_device is registered

Don't use netdev_info and friends before the net_device is registered.
This avoids ugly messages like
"meson8b-dwmac c9410000.ethernet (unnamed net_device) (uninitialized):
Enable RX Mitigation via HW Watchdog Timer"

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>